Android: moving update check to thread pool

Currently, AsyncTask.execute() defaults to the SERIAL_EXECUTOR. This
exector is good for preventing concurrency errors since it guarantees
serial execution, but bad for performance since the entire app shares
this single queue.

It looks like this callsite can use the THREAD_POOL_EXECUTOR instead,
since this use doesn't appear to rely on the concurrency guarantees that
SERIAL_EXECUTOR provides.

Bug: 869907
Change-Id: I09f794fe73c706019ce42f16b505d9a1ee1757ea
Reviewed-on: https://chromium-review.googlesource.com/1159268Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580169}

chrome/android/java/src/org/chromium/chrome/browser/omaha/UpdateMenuItemHelper.java

@@ -132,7 +132,8 @@ public class UpdateMenuItemHelper {
                 activity.onCheckForUpdate(mUpdateAvailable);
                 recordUpdateHistogram();
             }
-        }.execute();
+        }
+                .executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     /**