GitLab

Remove HpackHuffmanAggregator header, implementation, test, and build rules.
Obsolete Net.SpdyHpackEncoddeCharacterFrequency histogram.  As the HTTP/2
standard is finalized, there is no longer a need to gather character
distribution statistics.

BUG=414758

Review URL: https://codereview.chromium.org/1017853004

Cr-Commit-Position: refs/heads/master@{#322467}