Fix the IconLoadingPipeline::OnReadWebAppIcon crash issue.

CL:2454920 could fix the issue. This CL is used to make it better,
because SkBitmap could be empty:
https://source.chromium.org/chromium/chromium/src/+/master:chrome/browser/web_applications/web_app_icon_manager.cc;drc=a450ab794839b63f9c2ba982f738945f90e9fee2;l=422

So for the empty SkBitmap, we can return an empty imageskia, and don't
need to resize or add the ImageSkiaRep.

BUG=1137484

Change-Id: I0f27c75885a9ced8fbc2f086729b0d58b42be725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2470821
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817245}

chrome/browser/apps/app_service/app_icon_factory.cc

@@ -953,7 +953,7 @@ void IconLoadingPipeline::OnReadWebAppIcon(
       ++it;
     }
 
-    if (it == icon_bitmaps.end()) {
+    if (it == icon_bitmaps.end() || it->second.empty()) {
       MaybeApplyEffectsAndComplete(gfx::ImageSkia());
       return;
     }