Update compositor IsActive when view visibility changes.
Update compositor IsActive when view visibility changes.

When WebView enables blink page visibility API, View visibility factors
into calculating WebView's visibility and the compositor's "is_active" state.

WebView apps might put WebView invisible when loading a URL and set
the view to be visible onPageFinished. WebView needs to make the
compositor active when that happens.

BUG=520089

Review URL: https://codereview.chromium.org/1381913004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#352483}

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