Don't use border stroke for live caption

The difference between NO_SHADOW and NO_ASSETS is that we still draw a
border stroke for the former. Since live caption uses the same
background color for different themes, it should not use a themed system
border stroke. This CL removes the border stroke which is not visible
anyway.

Bug: 940027
Change-Id: I249191cfd51e82ea835e7e1f5997ac98a9c33bab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2590706Reviewed-by: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836888}

chrome/browser/ui/views/accessibility/caption_bubble.cc

@@ -195,7 +195,7 @@ CaptionBubble::CaptionBubble(views::View* anchor,
                              base::OnceClosure destroyed_callback)
     : BubbleDialogDelegateView(anchor,
                                views::BubbleBorder::FLOAT,
-                               views::BubbleBorder::Shadow::NO_SHADOW),
+                               views::BubbleBorder::Shadow::NO_ASSETS),
       destroyed_callback_(std::move(destroyed_callback)),
       ratio_in_parent_x_(kDefaultRatioInParentX),
       ratio_in_parent_y_(kDefaultRatioInParentY),