MSE-in-workers prep: Simplify MediaSource::Seekable()

Reduce potential cross-thread complexity when MediaSource is owned by a
worker context by eliminating cross-object calls from
MediaSource::Seekable() through HTMLMediaElement::buffered() and back to
MediaSource::Buffered().

BUG=878133

Change-Id: I0f89d5ba91ce20fdaea96a505c1ff5943f260f46
Reviewed-on: https://chromium-review.googlesource.com/c/1415438
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624243}
1 file changed