Introduce HTTP2_RELOADABLE_FLAG_COUNT.

Introduce HTTP2_RELOADABLE_FLAG_COUNT and remove platform-specific
prefix.  This way HTTP2_RELOADABLE_FLAG_COUNT syntax will be identical
across platforms.

Also remove unused HTTP2_FLAG_COUNT_N and HTTP2_CODE_COUNT* macros.

This CL merges internal change 224404468 by bnc.

Bug: 488484

Change-Id: Ib1c29d1c59101af7a60c85805ed53ae612212b25
Reviewed-on: https://chromium-review.googlesource.com/c/1366576
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Commit-Queue: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614678}
5 files changed