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These CLs implement a simple frame recording and fetch "extension" for the Chromoting protocol, to allow sample sessions to be captured for performance testing.

The three main classes introduced are:
- HostExtensionSessionManager
  This pulls logic for handling Chromoting extensions out of
  ClientSession instances into a dedicated manager class.
  It also introduces hooks through which extensions can
  wrap or replace the Chromoting video encoder or capturer.
- VideoFrameRecorder
  This allows a VideoEncoder to be wrapped to return a new
  encoder that will optionally copy & deliver frames to the
  VideoFrameRecorder before supplying them to the real
  encoder.
- VideoFrameRecorderHostExtension
  This extension uses a VideoFrameRecorder to allow a
  connected client to start/stop recording, and to retrieve
  the resulting frame data.

Original CLs:
crrev.com/402233003
crrev.com/339073002
crrev.com/372943002
crrev.com/462503002

BUG=260879

Review URL: https://codereview.chromium.org/468613002

Cr-Commit-Position: refs/heads/master@{#292541}