Allow trailing headers with duplicate keys. Protected by FLAGS_quic_reloadable_flag_quic_allow_dupl…

Allow trailing headers with duplicate keys. Protected by FLAGS_quic_reloadable_flag_quic_allow_duplicate_trailers

QUIC currently treats trailers with duplicate header keys as a critical failure
(causing a CONNECION_CLOSE with QUIC_INVALID_HEADERS_STREAM_DATA). See
b/62770138.

This doesn't seem to be implementing the spec, as RFC 7541 says:

"The dynamic table can contain duplicate entries (i.e., entries with the same
name and same value). Therefore, duplicate entries MUST NOT be treated as an
error by a decoder."

In addition, it doesn't look like our spdy/http2 implementation does any sort
of similar validation check.

This CL makes QUIC more permissive of trailing headers, brings it in line with
our HTTP/2 implementation, and (hopefully) fixes b/62770138.

Merge internal change: 159476633

R=rch@chromium.org

[wangyix1, 2017-06-28 19:09:57]

The CQ bit was checked by wangyix@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-06-28 19:10:29]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-06-28 19:14:46]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-06-28 19:14:47]

Dry run: Try jobs failed on following builders:
  android_clang_dbg_recipe on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_clan...)
  android_compile_dbg on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_comp...)
  android_cronet on master.tryserver.chromium.android (JOB_FAILED,
https://build.chromium.org/p/tryserver.chromium.android/builders/android_cron...)
  chromium_presubmit on master.tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)
  ios-simulator on master.tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/ios-simulator/bui...)

[wangyix1, 2017-06-28 21:39:37]

The CQ bit was checked by wangyix@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-06-28 21:40:20]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-06-28 23:40:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-06-28 23:40:55]

Dry run: This issue passed the CQ dry run.

[Ryan Hamilton, 2017-06-29 02:52:59]

lgtm