🦁Show favicons retrieved from cache immediately, preventing flashing

Before, the favicon completion was always called asynchronously,
leading to the original icon appearing for a split second when a
favicon was found in the cache. Now, that completion will be displayed
instantly.

The asynchronicity was there to simplify the FaviconRetriever logic
(the completion was always called asynchronously), but that isn't
actually super important.

Bug: 964918
Change-Id: I30b7d2f8d9fe0ed9324e48eb994ce677398b388b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626861
Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662595}

ios/chrome/browser/ui/omnibox/popup/favicon_retriever.h

@@ -12,7 +12,8 @@ class GURL;
 // favicon retrieving logic from the View layer.
 @protocol FaviconRetriever <NSObject>
 // Fetches favicon given a page URL.
-// |completion| is guaranteed to only be called asynchronously on main thread.
+// |completion| is guaranteed to only be called on main thread, but could be
+// called at any time, even before this returns.
 // It might never be called if the favicon is not available.
 // It might be called multiple times per request.
 - (void)fetchFavicon:(GURL)pageURL completion:(void (^)(UIImage*))completion;

ios/chrome/browser/ui/omnibox/popup/omnibox_popup_mediator.mm

@@ -195,9 +195,7 @@ const CGFloat kOmniboxIconSize = 16;
   // Only use cached attributes when they are a non-default icon. Never show
   // monograms or default globe icon.
   if (cachedAttributes.faviconImage && !cachedAttributes.usesDefaultImage) {
-    dispatch_async(dispatch_get_main_queue(), ^() {
       completion(cachedAttributes.faviconImage);
-    });
   }
 }