Use skia.mojom.BitmapN32 in VR service 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: I6e88c4e333587614083ed8213533241a09ee63f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2575446
Auto-Submit: danakj <danakj@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833956}

device/vr/public/mojom/vr_service.mojom

@@ -111,7 +111,7 @@ enum XRInteractionMode {
 };
 
 struct XRTrackedImage {
-  skia.mojom.BitmapWithArbitraryBpp bitmap;
+  skia.mojom.BitmapN32 bitmap;
   gfx.mojom.Size size_in_pixels;
   float width_in_meters;
 };