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Begins relaxation with a narrow case: addSourceBuffer("video/mp4") and
changeType("video/mp4") may now succeed, though any actual media
appended to them later must be encrypted HEVC content, and even then,
only on ChromeOS. Later changes will expand the relaxation of
codec-specificity further beyond this.

Specifically, this change:
1) Updates MediaSource and SourceBuffer to continue to require
   codec-specificity for MediaSource.isTypeSupported(), but includes a
   short-term special casing for addSourceBuffer and changeType to
   support just "video/mp4" on ChromeOS with HEVC EME support.

2) Updates Chromium StreamParserFactory to let parser creation occur
   with assumed eventual HEVC codec track when just "video/mp4" is used
   as addSourceBuffer() or changeType() parameter, but only on ChromeOS
   with HEVC EME support.

3) Updates Chromium SourceBufferState to require an HEVC track reported
   by the StreamParser to be disallowed with DCHECK failure if
   BUILDFLAG(ENABLE_PLATFORM_HEVC) is not true, as a protection against
   possible regression later in StreamParser function.

4) Updates Chromium SourceBufferState to require an HEVC track reported
   by the StreamParser to be encrypted (otherwise parse failure) if
   BUILDFLAG(USE_CHROMEOS_PROTECTED_MEDIA) is true.

5) Until wider relaxation is done, this change inserts a hardcoded
   hevc codec string in SourceBufferState's |expected_codecs| when just
   "video/mp4" is used for addSourceBuffer or changeType on
   ChromeOS+HEVC, enabling re-use of existing codec-specificity checks
   in SourceBufferState.

BUG=chromium:535738,b:153111783

Change-Id: I353cdbdb3c1fdafbeacacc6b7c8c39430be7d798
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2587494Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837916}