Refactor screen_dimmer to share code and shrink Shell interface. DimWindow : a window that implement dimming. You can just add it to container and let it handle its state. ScreenDimmer : an object that keeps the state consistent across multiple root windows. It's owned by the primary container and gets destroyed when the container is destroyed.

* Added wm::SetWindowVisiblityChangesAnimated to ui/wm/core/visibility_controller.h, to animate individual window's visibility change. The dim layer can be added to root (or any other container), and we probably don't want to enable animation for all children in such case.

* Use solid color layer for modal background. it's more efficient.

* Use WindowTracker to keep track of the non toplevel windows in RootWindowController::CloseChildWindows(). This was causing asan failure reported in crbug.com/547178

BUG=478438,547178
TBR=sky@chromium.org

TEST=updated existing screen_dimmer_unittest, VisibilityControllerTest.SetWindowVisibilityChangesAnimated, asan bot fails without change in RootWindowController::CloseChildWindows

Review URL: https://codereview.chromium.org/1426573004

Cr-Commit-Position: refs/heads/master@{#356359}

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