IconLoadingPipeline: Allow for missing icons

When an app is newly uninstalled, we might attempt to read icons that
are no longer present.

This CL should avoid some flakes in browser tests, i.e.
[app_icon_factory.cc(956)] Check failed: it != icon_bitmaps.end().
https://bugs.chromium.org/p/chromium/issues/detail?id=1119807#c14

Bug: 1119807
Change-Id: I05d0c0c63f26f1106b9f4674f9be350eb5c3a530
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2454920Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Commit-Queue: Nancy Wang <nancylingwang@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Auto-Submit: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814577}

chrome/browser/apps/app_service/app_icon_factory.cc

@@ -953,7 +953,11 @@ void IconLoadingPipeline::OnReadWebAppIcon(
       ++it;
     }
 
-    DCHECK(it != icon_bitmaps.end());
+    if (it == icon_bitmaps.end()) {
+      MaybeApplyEffectsAndComplete(gfx::ImageSkia());
+      return;
+    }
+
     SkBitmap bitmap = it->second;
 
     // Resize |bitmap| to match |icon_scale|.