GitLab

Add FLAGS_chromium_http2_flag_spdy_use_hpack_decoder3.

Modify SpdyFramer::GetHpackDecoder to create an HpackDecoder3 if this
new flag is set.

Extend the parameterized tests in quic_headers_stream_test.cc and
spdy_framer_test.cc to include testing of HpackDecoder3.

This CL lands server change 131193186 by jamessynge and 132112978 by rch.

BUG=488484

Review-Url: https://codereview.chromium.org/2644683002
Cr-Commit-Position: refs/heads/master@{#444820}