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This helps clients to decide when to rasterize ui::ImageModel.
For instance, if a new ImageModel is same with the old one,
they can skip rasterization.

Bug: 1048901
Change-Id: I72ba2bdc71d9cde576f99fe5cbf3db994a8b6837
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303747Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sang Woo Ko <sangwoo108@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790642}