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WebMIDI backend implementations have used double as a time type.
As base::TimeTicks is available in Blink now, it's better fit to use
as the timestamp.

Change-Id: Ide762f3a8dd503b3bd77eae9077f5ecde18e3796
Reviewed-on: https://chromium-review.googlesource.com/892726Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533988}