Android: make loadNextTab AsyncTask explicit on SERIAL_EXECUTOR

We might be changing the default execute(), so we want to make this an
explicit call to the SERIAL_EXECUTOR.

Bug: 869907
Change-Id: Ifd7cff98b7714b664fdc6cb30bc0d0f7b1a3be4a
Reviewed-on: https://chromium-review.googlesource.com/1162076
Commit-Queue: Sam Maier <smaier@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580559}

chrome/android/java/src/org/chromium/chrome/browser/tabmodel/TabPersistentStore.java

@@ -1266,7 +1266,7 @@ public class TabPersistentStore extends TabPersister {
         } else {
             TabRestoreDetails tabToRestore = mTabsToRestore.removeFirst();
             mLoadTabTask = new LoadTabTask(tabToRestore);
-            mLoadTabTask.execute();
+            mLoadTabTask.executeOnExecutor(AsyncTask.SERIAL_EXECUTOR);
         }
     }