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}