Add partial magnifier to ash palette.

The partial magnifier will magnify a small portion of the screen, similar to a spyglass.

TEST=./out/Release/ash_unittests --gtest_filter=PartialMagnificationControllerTest.*
TBR=oshima@chromium.org

BUG=616112

Review-Url: https://codereview.chromium.org/2239553002
Cr-Commit-Position: refs/heads/master@{#414124}
bool CanHandleShowMessageCenterBubble() {
  RootWindowController* controller =
      RootWindowController::ForTargetRootWindow();
  StatusAreaWidget* status_area_widget =
      controller->shelf_widget()->status_area_widget();
  return status_area_widget &&
         status_area_widget->web_notification_tray()->visible();
}

bool CanHandleShowMessageCenterBubble() {
  RootWindowController* controller =
      RootWindowController::ForTargetRootWindow();
  StatusAreaWidget* status_area_widget =
      controller->shelf_widget()->status_area_widget();
  return status_area_widget &&
         status_area_widget->web_notification_tray()->visible();
}
