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Once we have shipped this for a release this feature flag can be removed.
I've tested Windows and the issues causing the spurious ET_MOUSE_EXITED
don't exist anymore so I believe this is safe to ship in this fashion.

BUG=450631,240300,352106,386896,569998,575208

Review-Url: https://codereview.chromium.org/2618063002
Cr-Commit-Position: refs/heads/master@{#442076}