Android: moving ReadBookmarksTask 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: Ied2b63aaf8256033ada85c7d3367db0c4082b454
Reviewed-on: https://chromium-review.googlesource.com/1161084Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582723}

chrome/android/java/src/org/chromium/chrome/browser/partnerbookmarks/PartnerBookmarksReader.java

@@ -124,7 +124,7 @@ public class PartnerBookmarksReader {
             assert false : "readBookmarks called after nativeDestroy.";
             return;
         }
-        new ReadBookmarksTask().execute();
+        new ReadBookmarksTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     /**