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This is the preparing for CL:2305816. Add RawIconPngData to fetch the
foreground and background icon images.

BUG=1083331

Change-Id: I16112cebffc0296852c585c05b9047ce5926dcd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307010Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790654}