Use skia.mojom.BitmapN32 in FrameSinkVideoCaptureOverlay mojoms

These bitmaps are safer for transport from untrustworthy sources since
all bitmaps should be in N32 format and the browser can make bad
assumptions as a result.

R=dcheng@chromium.org

Bug: 1144462
Change-Id: Ieb4843b337ad2b26d85c49d7630f5966d5bdb378
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580544
Auto-Submit: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834979}

services/viz/privileged/mojom/compositing/frame_sink_video_capture.mojom

@@ -153,7 +153,7 @@ interface FrameSinkVideoCaptureOverlay {
   // content (e.g., 0.0 refers to the top or left edge; 1.0 to just after the
   // bottom or right edge). Pass empty |bounds| to temporarily hide the overlay
   // until a later call to SetBounds().
-  SetImageAndBounds(skia.mojom.BitmapWithArbitraryBpp image,
+  SetImageAndBounds(skia.mojom.BitmapN32 image,
                     gfx.mojom.RectF bounds);
 
   // Changes the bounds of the previously-set image, showing the overlay if