Replace MessageLoopProxy usage with ThreadTaskRunnerHandle in icon_loader files of chrome/browser module.

MessageLoopProxy is deprecated.
This basically does a search and replace of:

MessageLoopProxy::current() -> ThreadTaskRunnerHandle::Get().

MessageLoopProxy -> SingleThreadTaskRunner

BUG=391045

Review URL: https://codereview.chromium.org/1110313002

Cr-Commit-Position: refs/heads/master@{#327470}
diff --git a/chrome/browser/icon_loader_mac.mm b/chrome/browser/icon_loader_mac.mm
index 0757d1f..6abeb47 100644
--- a/chrome/browser/icon_loader_mac.mm
+++ b/chrome/browser/icon_loader_mac.mm
@@ -57,6 +57,6 @@
     }
   }
 
-  target_message_loop_->PostTask(FROM_HERE,
+  target_task_runner_->PostTask(FROM_HERE,
       base::Bind(&IconLoader::NotifyDelegate, this));
 }