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Ideally we want to remove the fallback ticking of webview entirely, but
in the interim we should move it to the compositor thread in order to
avoid the sync IPC for the software draw as well as the shared memory
allocation.

BUG=431166

Review URL: https://codereview.chromium.org/1782093002

Cr-Commit-Position: refs/heads/master@{#381688}