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In order to save memory and upload time, images which are
going to be drawn at less than their native resolution may
be uploaded at a smaller scale. This change adds additional
caching and upload logic to allow this.
See https://goo.gl/0zCd9Z for a complete description.

BUG=623688
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel;tryserver.blink:mac_blink_rel;tryserver.blink:win_blink_rel

Committed: https://crrev.com/bc8acf2632042e5e93982088d5b7dcb7f1c0d168
Review-Url: https://codereview.chromium.org/2042133002
Cr-Original-Commit-Position: refs/heads/master@{#402377}
Cr-Commit-Position: refs/heads/master@{#402694}