Android: Webapp register and get splash screen AsyncTasks 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 these callsites can use the THREAD_POOL_EXECUTOR instead,
since these usages don't appear to rely on the concurrency guarantees
that SERIAL_EXECUTOR provides.

Bug: 869907
Change-Id: Iaf227dc516d1b2c850b9b17ce5d43302ab6532b2
Reviewed-on: https://chromium-review.googlesource.com/1161968
Reviewed-by: agrieve <agrieve@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580780}

base/test/android/junit/src/org/chromium/base/test/asynctask/BackgroundShadowAsyncTask.java

@@ -13,6 +13,7 @@
 import org.chromium.base.AsyncTask;
 
 import java.util.concurrent.Callable;
+import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
@@ -44,6 +45,12 @@ public AsyncTask<Result> call() throws Exception {
         }
     }
 
+    @Override
+    @Implementation
+    public final AsyncTask<Result> executeOnExecutor(Executor e) {
+        return execute();
+    }
+
     @Override
     @Implementation
     public final Result get() {

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappDataStorage.java

@@ -183,7 +183,7 @@ protected final void onPostExecute(Bitmap result) {
                 callback.onDataRetrieved(result);
             }
         }
-                .execute();
+                .executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     /**

chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappRegistry.java

@@ -136,7 +136,7 @@ protected final void onPostExecute(WebappDataStorage storage) {
                 if (callback != null) callback.onWebappDataStorageRetrieved(storage);
             }
         }
-                .execute();
+                .executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
     }
 
     /**