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When porting StartDragging to Mojo in https://crrev.com/c/2320268, the
image contents weren't being set in the drop data. This resulted in a
link being saved on Windows and an error shown on Linux.

Bug: 1039255, 1132953
Change-Id: I2d21e0424c89306d5df8caf940e93999d16f78f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2444996Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Cr-Commit-Position: refs/heads/master@{#813645}