GMC: Prevent white pixels in the corners of media notifications

This CL sets the MediaNotificationListView's background to transparent
so that white pixels don't peek through in the corners of the dialog.

Bug: 1006234
Change-Id: I3cea0c32d4b9ba238d327e219e052265e06a1847
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1816974Reviewed-by: Jazz Xu <jazzhsu@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699855}

chrome/browser/ui/views/global_media_controls/media_notification_list_view.cc

@@ -15,6 +15,7 @@ constexpr int kMediaListMaxHeight = 478;
 }  // anonymous namespace
 
 MediaNotificationListView::MediaNotificationListView() {
+  SetBackgroundColor(SK_ColorTRANSPARENT);
   SetContents(std::make_unique<views::View>());
   contents()->SetLayoutManager(std::make_unique<views::BoxLayout>(
       views::BoxLayout::Orientation::kVertical));