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This CL addresses the issue where when the timeline slider is focused
while the media is playing, Narrator will announce 'Elapsed time,
Elapsed time, ..." repeatedly. (NVDA will repeat the whole sentence
which is also not ideal)

With this change, the behavior is now changed to announce the
current/elapsed time only once when the slider gets focused.

Note
1. only Aria updates triggered by the timer are suppressed, so
user interactions that would move timeline slider, ex mouse clicks,
left/right keys etc, would still trigger current/elapsed time utterance.

2. we can't use aria-live:off to suppress aria since according to
the spec [1], aria-live:off doesn't apply to areas that are being focused.

[1] https://www.w3.org/TR/wai-aria-1.1/#aria-live

Bug: 1054885
Change-Id: I5519793a3d53f1982733f49fd4db882326f5c2f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067381
Commit-Queue: Alex Keng <shihken@microsoft.com>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744920}