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This avoids re-encoding thumbnail images by requesting compressed image
data directly from ThumbnailImage.

Additionally, this changes the data URI generating code to reduce usage
of temporary std::strings.

Bug: 991393
Change-Id: Ia5cc679c383e5ef4a084c5b220492c824124b7d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836146
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705166}