Uses the `DataFrameAssembler` in `Http2Dispatcher`.
This is another step toward simplification/optimization of the write path. The new implementation is 1 line longer than the old one, but `DataFrameAssembler` is simpler than `Http2DataFrameBuilder`, and the code is easier to reason about.
Tested in three batches:
```
blaze test --test_arg=--gfe2_reloadable_flag_http2_data_frame_assembler \
--test_arg=--undefok=gfe2_reloadable_flag_http2_data_frame_assembler \
//gfe/gfe2/http2:all //gfe/gfe2/http2/e2e:all //gfe/gfe2/vtl/... //gfe/gfe2/e2e/...
```
http://sponge2/f82a0a1d-7ba9-4812-b2df-d325dc1abc03
```
$ blaze test --test_arg=--gfe2_reloadable_flag_http2_data_frame_assembler \
--test_arg=--undefok=gfe2_reloadable_flag_http2_data_frame_assembler \
//gfe/e2e/...
```
http://sponge2/578edeee-efa3-4e42-87a2-e5344bf6ea6c
```
$ blaze test --test_arg=--gfe2_reloadable_flag_http2_data_frame_assembler \
--test_arg=--undefok=gfe2_reloadable_flag_http2_data_frame_assembler \
//gfe/gfe2/cloud/... //gfe/gfe2/dos/...
```
http://sponge2/a0fca691-7800-477a-be0d-2957f412095f
Protected by FLAGS_gfe2_reloadable_flag_http2_data_frame_assembler.
PiperOrigin-RevId: 662274008
diff --git a/quiche/http2/core/array_output_buffer.h b/quiche/http2/core/array_output_buffer.h
index 6fc9727..2dd3c39 100644
--- a/quiche/http2/core/array_output_buffer.h
+++ b/quiche/http2/core/array_output_buffer.h
@@ -30,6 +30,7 @@
size_t Size() const { return current_ - begin_; }
char* Begin() const { return begin_; }
+ char* Current() const { return current_; }
// Resets the buffer to its original state.
void Reset() {
