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This feature is guarded by --chromium_flag_http2_varint_decode_64_bits.

Before this change, HpackVarintDecoder only decoded integers up to 2^28
+ 2^prefix_length - 2, where prefix_length is between 3 and 7.  Trying
to decode larger integers resulted in an error.  After this change,
HpackVarintDecoder decodes any integer that can be represented on 64
bits (unsigned).  Any input that HpackVarintDecoder decoded before this
change is still decoded.

Decoded integers are used to address entries in the static and dynamic
tables, to specify string lengths, and to update the size limit of the
dynamic table, both in HTTP/2 and in QUIC.  For all of these values
there is already a limit in place that is much lower than 2^28
(otherwise a malicious client could waste hundreds of megabytes of
server memory; and if there is no such limit in place, then we already
have a problem), so making the decoder decode larger values should not
change behavior at higher levels.

To merge this feature, this change also adds an HTTP/2 platform API for
flags,  maintaining parity with SPDY and QUIC (and keeping the
separation between SPDY and HTTP/2 for ease of merging to Chromium).
This change then uses this platform API in the related files referencing
the feature flag and adds a presubmit check.

This change borrows heavily from the pre-existing SPDY and QUIC platform
API for flags.

This CL lands server changes 216463549 and 221087569 by bnc, and
218390247 by diannahu.

BUG=488484

Change-Id: Ie0788f117aba9c6d41fc31148e28c9b49e122602
Reviewed-on: https://chromium-review.googlesource.com/c/1310613Reviewed-by: Bence Béky <bnc@chromium.org>
Commit-Queue: Dianna Hu <diannahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607349}