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This CL adds a WasEverAudible method to WebContents which returns true
if audio has played since the last navigation. This is then used by the
Page Info bubble to decide whether or not to show the sound content
setting.

Bug: 779878
Change-Id: I66453161dfe17df6868e66f11e7fce0b3c3a87b8
Reviewed-on: https://chromium-review.googlesource.com/747069Reviewed-by: Lucas Garron <lgarron@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513025}