Use uint64_t for varint.

gfe-relnote: Change two types from uint32_t to uint64_t.  Not flag protected.

Motivation: https://crbug.com/916990.

HpackVarintDecoder::value() has a return type of uint64_t.  This was returned by
HpackEntryTypeDecoder::varint() as a uint32_t, this CL changes that to uint64_t.
Outside tests HpackEntryTypeDecoder::varint() is called in
HpackEntryDecoder::Start(), where it is immediately passed to
HpackEntryDecoderListener::OnIndexedHeader(), which takes a size_t.

The other HpackEntryTypeDecoder::varint() production call site is
HpackEntryDecoder::DispatchOnType(), where a local variable |varint| is assigned
to its return value.  This CL changes |varint| type from uint32_t to uint64_t.
It is then passed to three methods:
HpackEntryDecoderListener::OnIndexedHeader(),
HpackEntryDecoderListener::OnStartLiteralHeader(), and
HpackEntryDecoderListener::OnDynamicTableSizeUpdate().  These all take size_t.

Every platform that GFE cares about has a 64 bit wide size_t.  So all this CL
does is remove a 64 bit to 32 bit to 64 bit integer conversion along a small
number of paths.

PiperOrigin-RevId: 226346424
Change-Id: I71f50f71e6d1702845b3e76a7427428c28aa6984

http2/hpack/decoder/hpack_entry_decoder.cc

diff --git a/http2/hpack/decoder/hpack_entry_decoder.cc b/http2/hpack/decoder/hpack_entry_decoder.cc
index 89562a9..b227b18 100644
--- a/http2/hpack/decoder/hpack_entry_decoder.cc
+++ b/http2/hpack/decoder/hpack_entry_decoder.cc
@@ -192,7 +192,7 @@
 
 bool HpackEntryDecoder::DispatchOnType(HpackEntryDecoderListener* listener) {
   const HpackEntryType entry_type = entry_type_decoder_.entry_type();
-  const uint32_t varint = entry_type_decoder_.varint();
+  const uint64_t varint = entry_type_decoder_.varint();
   switch (entry_type) {
     case HpackEntryType::kIndexedHeader:
       // The entry consists solely of the entry type and varint. See:

http2/hpack/decoder/hpack_entry_type_decoder.h

diff --git a/http2/hpack/decoder/hpack_entry_type_decoder.h b/http2/hpack/decoder/hpack_entry_type_decoder.h
index 1c0f2ac..79898be 100644
--- a/http2/hpack/decoder/hpack_entry_type_decoder.h
+++ b/http2/hpack/decoder/hpack_entry_type_decoder.h
@@ -38,7 +38,7 @@
 
   // Returns the decoded variable length integer. Only call if the
   // preceding call to Start or Resume returned kDecodeDone.
-  uint32_t varint() const { return varint_decoder_.value(); }
+  uint64_t varint() const { return varint_decoder_.value(); }
 
   Http2String DebugString() const;