Don't require screen updates for WKWebView snapshotting
Don't require screen updates for WKWebView snapshotting

Calling drawViewHierarchyInRect:afterScreenUpdates: with YES
disturbs the global animation state, causing videos to play back
out of sync with audio. Use NO instead, which may miss recent
updates, but appears to work in practice on iOS 9.

BUG=546773
TEST=In WKWebView mode, videos on iPad should not play with sound out of
sync with video. Features based on snapshots (e.g., phone tab switcher)
should still display snapshots correctly.

Review URL: https://codereview.chromium.org/1410293010

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359356}

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