Clip docked magnifier layer to its bounds

The actual content rect of magnified container might be larger than its
bounds. To avoid this extra content becoming visible in the docked
magnifier, clip magnifier layer to its bounds.

BUG=947565

Change-Id: I185df56c12a2d45118b5cf35ce6240ac2b5a5fb4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717540Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681443}

ash/magnifier/docked_magnifier_controller_impl.cc

@@ -743,6 +743,13 @@ void DockedMagnifierControllerImpl::CreateMagnifierViewport() {
   //    and magnified.
   viewport_magnifier_layer_ =
       std::make_unique<ui::Layer>(ui::LAYER_SOLID_COLOR);
+  // There are situations that the content rect for the magnified container gets
+  // larger than its bounds (e.g. shelf stretches beyond the screen to allow it
+  // being dragged up, or contents of mouse pointer might go beyond screen when
+  // the pointer is at the edges of the screen). To avoid this extra content
+  // becoming visible in the magnifier, magnifier layer should clip its contents
+  // to its bounds.
+  viewport_magnifier_layer_->SetMasksToBounds(true);
   viewport_layer->Add(viewport_magnifier_layer_.get());
   viewport_layer->SetMasksToBounds(true);