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There are 300 QPS for tiles now, not sure why is it happening.
Adding more restrictions are tile fetching.

Change-Id: Ie513364d0a072abc3f3d92ae1f09b404bf5df161
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601259Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#839471}