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When the user selects a background image actually update the
page background. The choice of background is stored in Preferences.
A new preference is added for this purpose: kNTPCustomBackgroundURL.
This is exposed to the NTP javascript through ThemeBackgroundInfo.
Add a NewTabPageBinding to update these preferences when an
image is selected by propagating the change to the InstantService
where the browser profile is available. The InstantService then
notifies all New Tab Pages of the change.

Bug: 839152
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ib6f74f1f3e9dda6a3352833662db373ec1358769
Reviewed-on: https://chromium-review.googlesource.com/1081651
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565320}