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Add SeekTo action that seeks to a position from the
beginning of the media.

Also adds a ScrubTo action that may be supported by
some media sessions. It is used for scrubbing on a
position scrubber. The client should call ScrubTo
while the user is scrubbing and then call SeekTo to
release the scrub.

BUG=977375

Change-Id: If926271b1aeebee7b3ef6529962213e1f2b0fde7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693171
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Sean Topping <seantopping@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676951}