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This implements an append option so we can append images to an existing
clipboard data object. This allows us to preserve existing clipboard
metadata and copy images to the system clipboard.

Bug: 827333
Change-Id: I89062055d21f252274c6a77cc72cfaf2eed3598a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463851Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Alexander Nohe <nohe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821176}