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In order to be able to use it in places other than content (e.g. handles
for unified touch selection controller), ImageWindowDelegate is moved to
a new ui module called aura_extra.

Also, to make it more useful, ImageWindowDelegate can now be customized
by setting background color and image offset.

COLLABORATOR=mfomitchev
BUG=399721

Review URL: https://codereview.chromium.org/880703002

Cr-Commit-Position: refs/heads/master@{#315344}