Use skia.mojom.BitmapN32 in ContentIndexService 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: Id5ead0d99d5cb2656416086433c6151c14b540a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2572897Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833954}

third_party/blink/public/mojom/content_index/content_index.mojom

@@ -83,7 +83,7 @@ interface ContentIndexService {
 
   Add(int64 service_worker_registration_id,
       ContentDescription description,
-      array<skia.mojom.BitmapWithArbitraryBpp> icon,
+      array<skia.mojom.BitmapN32> icon,
       url.mojom.Url launchUrl)
       => (ContentIndexError error);
   Delete(int64 service_worker_registration_id, string id)