MSE: Add StreamParser buffer remuxing utility and tests

Introduces remuxing utility in stream_parser.h for later refactoring
into fine-grained usage by each stream parser. Adds associated
unit tests.
Modifies StreamParserBuffer so buffers are annotated by type of media
(audio/video/text) and track ID on creation.
This is another prerequisite to implementing MSE-compliant coded frame
processing algorithm in SourceState that requires muxed-in-decode-
timestamp sequence buffers from all media types in the segment
instead of the current sequence per each media type.

R=xhwang@chromium.org
BUG=249422, 341581
TEST=media unittests and MSE layout tests pass locally on Linux.

Review URL: https://codereview.chromium.org/149153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249634 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249634