Improves the unit testing for HardwareVideoEncoder and fixes bugs in it. The main added feature is support for dynamically switching between texture and byte buffer modes. Bug: webrtc:7760 Change-Id: Iaffe6b7700047c7d0f9a7b89a6118f6ff932cd9b Reviewed-on: https://webrtc-review.googlesource.com/2682 Commit-Queue: Sami Kalliomäki <sakal@webrtc.org> Reviewed-by: Magnus Jedvert <magjed@webrtc.org> Cr-Commit-Position: refs/heads/master@{#19963}