GitLab

Change drag and drop to also use callbacks for data fetching like how
clipboard does. Ensure callbacks are only invoked once with data
cached, and pending receive requests managed.

Files drag and drop will require this to allow file sharing to be done
inside FileHandler at the point where a client calls data_offer_receive,
rather than when FileHandler is currently invoked when data is offered.

This change provides performance improvements particularly for when an
image is requested multiple times from the clipboard since we now only
do the encoding once.

Bug: 1144138
Change-Id: I56c7d3b55a3365a591829313462af23e71e172b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2509833Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Auto-Submit: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824129}