Add SPS parsing to H265 parser

This adds parsing of SPS from the H265 bitstream. To keep the size down
a little, it doesn't handle VUI and the other extension data yet.
That'll be in the next CL. The design follows the existing H264Parser.

BUG=chrome:1141237,b:153111783
TEST=media_unittests pass

Change-Id: If489ddb5a31fcdb323773a3f530719f51878a897
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491925


Commit-Queue: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Jeffrey Kardatzke <jkardatzke@google.com>
Cr-Commit-Position: refs/heads/master@{#820067}