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Sometimes we get into situations where we get frequent UpdateTiles,
but no AppendQuads. As a result we can start accumulating tilings
(during pinch zoom for example), but we never free any of them, since
the code to free tilings happens only during AppendQuads. This patch
remedies this by ensuring that if AppendQuads doesn't clean up the
tilings, and we've added some since last time we ran UpdateTiles,
we clean them up.

R=danakj
BUG=452304

Review URL: https://codereview.chromium.org/881083002

Cr-Commit-Position: refs/heads/master@{#313428}