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Start of support for screen magnification on chromecast. Consists of:

   * A port of PartialMagnificationController from Ash to Cast. Note
     that the first patchset is a direct copy, with modifications in
     subsequent patch sets, so a diff with patchset 1 will show the
     modifications made.

   * A base interface (MagnificationController) for implementations,
     as we will probably port fullscreen magnification as well.

   * Installation of the PartialMagnificationController on the root
     window via the window manager, but currently disabled.

Bug: internal b/110481596
Bug: internal b/110532452
Test: manual and unit test
Change-Id: Ia72f9b662304f5b5b3864c7fdfa7012677bfe883
Reviewed-on: https://chromium-review.googlesource.com/1111904
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570046}