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Renames HTMLMediaSource (in core/html/media) to be MediaSource, and
renames MediaSource (in modules/mediasource) to be MediaSourceImpl. This
follows more recent practice in core/html/media. See also related code
review comment [1].

[1] http://crrev.com/c/2003402/9/third_party/blink/renderer/core/html/media/html_media_source.h#50

BUG=878133

Change-Id: I143718c2b9bf8a2e32ad29e31c15094ae32da3fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2065421Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743201}