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This makes the SingleThreadProxy match the behaviour of ThreadProxy and
ensures that when the embedder is doing Layout() if it changes things
that would cause a commit, those things will already happen in the
upcoming commit so it does not schedule a second commit afterward.

R=enne,vmpstr
BUG=466426

Review URL: https://codereview.chromium.org/1061993009

Cr-Commit-Position: refs/heads/master@{#326884}