GMC: Center artwork.

This CL center the artwork in Global Media Controls.

Bug: 1031133
Change-Id: Ibfa6b11beeff4720143922d3d4affe75cb3bcbe0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107600Reviewed-by: Tommy Steimel <steimel@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755182}

components/media_message_center/media_notification_background.cc

@@ -386,11 +386,17 @@ gfx::Rect MediaNotificationBackground::GetArtworkBounds(
     const views::View& owner) const {
   const gfx::Rect& view_bounds = owner.GetContentsBounds();
   int width = GetArtworkWidth(view_bounds.size());
+  int visible_width = GetArtworkVisibleWidth(view_bounds.size());
+
+  // This offset is for centering artwork if artwork visible width is smaller
+  // than artwork width.
+  int horizontal_offset = (width - visible_width) / 2;
 
   // The artwork should be positioned on the far right hand side of the
   // notification and be the same height.
   return owner.GetMirroredRect(
-      gfx::Rect(view_bounds.right() - width, 0, width, view_bounds.height()));
+      gfx::Rect(view_bounds.right() - width + horizontal_offset, 0, width,
+                view_bounds.height()));
 }
 
 gfx::Rect MediaNotificationBackground::GetFilledBackgroundBounds(