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This CL adds ImageIndex, which stores annotation of an image file for
quick access to raw data and embedded references.
Note that some functionality from EncodedView from original implementation
have been moved into ImageIndex.

Bug: 729154
Change-Id: I1d5353fc8093543f2be0400361affa8b57333924
Reviewed-on: https://chromium-review.googlesource.com/589809
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491835}