Reland "[Media History] Fix watchtime recording"

This is a reland of c5b00d82d8a218f28296851c4b31e73eb9795822

Original change's description:
> [Media History] Fix watchtime recording
>
> Media History does not always record watchtime
> because the callback might be called after
> the WebContentsObserver has been destroyed.
> This moves it to WebContentsDelegate and adds
> some tests.
>
> BUG=1051176
>
> Change-Id: Ic60066535e3039cee07b2d0994c97e0279bd36e0
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2051289
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
> Reviewed-by: Tommy Steimel <steimel@chromium.org>
> Commit-Queue: Becca Hughes <beccahughes@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#749888}

Bug: 1051176
Change-Id: I365af299c128d5c68e8387ff55691614da82be71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2105404
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750970}