Move blink::mojom::PageVisibilityState to content::PageVisibilityState.

This moves the mojom enum to be a standard c++ enum class in
content/public/browser/. It's used by chrome/browser and
content/browser, as well as content/shell/test_runner, though the
latter one will be removed.

This enum is no longer used by the renderer code nor sent over IPC so
not need to be in blink or as a mojo enum.

R=avi@chromium.org, dcheng@chromium.org
TBR=dcheng

Change-Id: I18a81513332338ae36fe789ab4d289061335c4c3
Bug: 908582
Reviewed-on: https://chromium-review.googlesource.com/c/1357507
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612883}
62 files changed