Use skia.mojom.BitmapN32 in ImageDownloader 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: I1d55e3df22e9b16427de44ba229a1ce05528c6d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572997Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833951}

third_party/blink/public/mojom/image_downloader/image_downloader.mojom

@@ -21,6 +21,6 @@ interface ImageDownloader {
                 uint32 max_bitmap_size,
                 bool bypass_cache)
       => (int32 http_status_code,
-          array<skia.mojom.BitmapWithArbitraryBpp> images,
+          array<skia.mojom.BitmapN32> images,
           array<gfx.mojom.Size> original_image_sizes);
 };