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Now that selection bounds are routed through the compositor, enable the feature
in Blink and hook the CompositorFrameMetadata-carried bounds into the existing
selection handle pipeline.  Also suppress the sending of
ViewHostMsg_SelectionBoundsChanged messages when the feature is enabled.

This patch depends directly on
https://codereview.chromium.org/454643002/.

BUG=135959

Review URL: https://codereview.chromium.org/359033002

Cr-Commit-Position: refs/heads/master@{#296218}