Convert unnamed enums in third_party/quiche and third_party/http2 to constexpr variables. Modern C++ style prefers `constexpr` variables over unnamed enums for defining compile-time constants (go/totw/140). This change replaces instances of unnamed enums defining constants across QUICHE and HTTP2 libraries with their `constexpr` equivalents. Unnamed enums defined within classes are replaced with `static constexpr` variables, while those outside classes are replaced with `inline constexpr` variables. PiperOrigin-RevId: 927430492

diff --git a/quiche/common/quiche_data_writer.h b/quiche/common/quiche_data_writer.h
index 94f8480..1060dbb 100644
--- a/quiche/common/quiche_data_writer.h
+++ b/quiche/common/quiche_data_writer.h

@@ -19,21 +19,17 @@
 
 // Maximum value that can be properly encoded using RFC 9000 62-bit Variable
 // Length Integer encoding.
-enum : uint64_t {
-  kVarInt62MaxValue = UINT64_C(0x3fffffffffffffff),
-};
+inline constexpr uint64_t kVarInt62MaxValue = UINT64_C(0x3fffffffffffffff);
 
 // RFC 9000 62-bit Variable Length Integer encoding masks
 // If a uint64_t anded with a mask is not 0 then the value is encoded
 // using that length (or is too big, in the case of kVarInt62ErrorMask).
 // Values must be checked in order (error, 8-, 4-, and then 2- bytes)
 // and if none are non-0, the value is encoded in 1 byte.
-enum : uint64_t {
-  kVarInt62ErrorMask = UINT64_C(0xc000000000000000),
-  kVarInt62Mask8Bytes = UINT64_C(0x3fffffffc0000000),
-  kVarInt62Mask4Bytes = UINT64_C(0x000000003fffc000),
-  kVarInt62Mask2Bytes = UINT64_C(0x0000000000003fc0),
-};
+inline constexpr uint64_t kVarInt62ErrorMask = UINT64_C(0xc000000000000000);
+inline constexpr uint64_t kVarInt62Mask8Bytes = UINT64_C(0x3fffffffc0000000);
+inline constexpr uint64_t kVarInt62Mask4Bytes = UINT64_C(0x000000003fffc000);
+inline constexpr uint64_t kVarInt62Mask2Bytes = UINT64_C(0x0000000000003fc0);
 
 // This class provides facilities for packing binary data.
 //

diff --git a/quiche/common/quiche_ip_address.h b/quiche/common/quiche_ip_address.h
index a6eeffd..ec18646 100644
--- a/quiche/common/quiche_ip_address.h
+++ b/quiche/common/quiche_ip_address.h

@@ -28,11 +28,9 @@
 class QUICHE_EXPORT QuicheIpAddress {
  public:
   // Sizes of IP addresses of different types, in bytes.
-  enum : size_t {
-    kIPv4AddressSize = 32 / 8,
-    kIPv6AddressSize = 128 / 8,
-    kMaxAddressSize = kIPv6AddressSize,
-  };
+  static constexpr size_t kIPv4AddressSize = 32 / 8;
+  static constexpr size_t kIPv6AddressSize = 128 / 8;
+  static constexpr size_t kMaxAddressSize = kIPv6AddressSize;
 
   // TODO(fayang): Remove Loopback*() and use TestLoopback*() in tests.
   static QuicheIpAddress Loopback4();

diff --git a/quiche/http2/adapter/data_source.h b/quiche/http2/adapter/data_source.h
index 589e768..2df11a2 100644
--- a/quiche/http2/adapter/data_source.h
+++ b/quiche/http2/adapter/data_source.h

@@ -14,7 +14,8 @@
 // TODO(birenroy): move the remaining constants.
 class QUICHE_EXPORT DataFrameSource {
  public:
-  enum : int64_t { kBlocked = 0, kError = -1 };
+  static constexpr int64_t kBlocked = 0;
+  static constexpr int64_t kError = -1;
 };
 
 // Represents a source of metadata frames for transmission to the peer.

diff --git a/quiche/http2/adapter/http2_visitor_interface.h b/quiche/http2/adapter/http2_visitor_interface.h
index fffd59a..7cff2bf 100644
--- a/quiche/http2/adapter/http2_visitor_interface.h
+++ b/quiche/http2/adapter/http2_visitor_interface.h

@@ -53,10 +53,8 @@
   Http2VisitorInterface& operator=(const Http2VisitorInterface&) = delete;
   virtual ~Http2VisitorInterface() = default;
 
-  enum : int64_t {
-    kSendBlocked = 0,
-    kSendError = -1,
-  };
+  static constexpr int64_t kSendBlocked = 0;
+  static constexpr int64_t kSendError = -1;
   // Called when there are serialized frames to send. Should return how many
   // bytes were actually sent. May return kSendBlocked or kSendError.
   virtual int64_t OnReadyToSend(absl::string_view serialized) = 0;