Use skia.mojom.BitmapN32 in PaintPreviewCompositor 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: Icc05e473bb85ee858781b0e40d20d727c8191a8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574786
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: danakj <danakj@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833955}

components/services/paint_preview_compositor/public/mojom/paint_preview_compositor.mojom

@@ -111,7 +111,7 @@ interface PaintPreviewCompositor {
   BitmapForSeparatedFrame(mojo_base.mojom.UnguessableToken frame_guid,
                           gfx.mojom.Rect clip_rect,
                           float scale_factor) =>
-    (BitmapStatus status, skia.mojom.BitmapWithArbitraryBpp? bitmap);
+    (BitmapStatus status, skia.mojom.BitmapN32? bitmap);
 
   // Starts the compositing process for |BitmapForMainFrame| calls using frame
   // data |request|. |status| will be negative on failure. If the root frame is
@@ -127,7 +127,7 @@ interface PaintPreviewCompositor {
   // |status|.
   // |BeginMainFrameComposite| must be called before this method.
   BitmapForMainFrame(gfx.mojom.Rect clip_rect, float scale_factor) =>
-    (BitmapStatus status, skia.mojom.BitmapWithArbitraryBpp? bitmap);
+    (BitmapStatus status, skia.mojom.BitmapN32? bitmap);
 
   // Sets the root frame of the compositor. Used for tracing and diagnostics.
   SetRootFrameUrl(url.mojom.Url url);