HPACK optimal Huffman code instrumentation and UMA collection.

Adds support for aggregating observed character counts in literals emitted by
a per-origin HPACK encoder. This is intended to approximate the literal
encodings which would be used were the user agent speaking HTTP/2 to the
origin. Character counts sampled and aggregated across a population will be used
to design an optimial static Huffman compression table for the HTTP/2 standard.

See design doc:
https://docs.google.com/a/google.com/document/d/1Y6pnBf8u24AHNFcA6GADn2XgHzfIvZiIN39Kf11raIY/edit#

BUG=368301

Review URL: https://codereview.chromium.org/243153003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267131 0039d316-1c4b-4281-b951-d872f2087c98
11 files changed