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This is the first patch in a series to make screenshots work in
mash. This first step is simplifying ui::ScreenshotGrabber into a
component that can be used without the filesystem. This moves file
writing to the chrome layer, switches to callbacks, and starts
separating ChromeScreenshotGrabber into the part that initiates a
screenshot (will be replaced with an ash side delegate that communicates
over mojo) and the part that accepts the data and writes it to the
filesystem (which will continue to exist and stay in chrome).

Change-Id: Ic2370e0d4426f51ffd09af8920304b98b8ebde0e
Bug: 706246
Reviewed-on: https://chromium-review.googlesource.com/894882Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Elliot Glaysher <erg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533848}