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This CL extracts the drag icon surface class from DragDropOperation.
It can be null and there is no need to create a SurfaceTreeHost for it
in such case. This is not possible currently, because DragDropOperation
inherits from SurfaceTreeHost. This CL extracts it into an internal
class and uses composition (instead of inheritance) to create the
IconSurface only when needed.

This patch does not cause behavioral changes.

Bug: 1102946
Change-Id: If7497cbe436ef8bdb5fb3a80a184c5ac136732db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311127
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825332}