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CL #1 of 2 for chromevox announcements (second part is for tab
navigation).

Add the following announcements:
-Entering capture mode
-Exiting capture mode
-Selecting the type (image/video)
-Selecting the source (window/region/fullscreen)
-Screenshot captured
-Recording starting
-Recording stopped

Bug: 1140597
Test: manual
Change-Id: I17c8d62de27507dfd4a9711201525fbd76b993cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2598056
Commit-Queue: Richard Chui <richui@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843917}