Allow NativeBackgrounTask to start in ServiceManagerOnly mode

NativeBackgroundTask now allows subclasses to indicate if they support
lite mode. If they do, the EmptyBrowserParts that it creates would also
signal the same to native.

Bug: 913480
Change-Id: I85bf27e9edd5b01cee7756c7a951cd95d3e9520e
Reviewed-on: https://chromium-review.googlesource.com/c/1370580Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615932}

chrome/android/java/src/org/chromium/chrome/browser/background_task_scheduler/NativeBackgroundTask.java

@@ -108,6 +108,10 @@ public abstract class NativeBackgroundTask implements BackgroundTask {
                 ThreadUtils.postOnUiThread(startWithNativeRunnable);
             }
             @Override
+            public boolean startServiceManagerOnly() {
+                return supportsServiceManagerOnly();
+            }
+            @Override
             public void onStartupFailure() {
                 ThreadUtils.postOnUiThread(rescheduleRunnable);
             }
@@ -134,6 +138,19 @@ public abstract class NativeBackgroundTask implements BackgroundTask {
         });
     }
 
+    /**
+     * Descendant classes should override this method if they support running in service manager
+     * only mode.
+     *
+     * TODO(https://crbug.com/913480): implement in a subclass once it can support running in
+     * ServiceManager only mode.
+     *
+     * @return if the task supports running in service manager only mode.
+     */
+    protected boolean supportsServiceManagerOnly() {
+        return false;
+    }
+
     /**
      * Method that should be implemented in derived classes to provide implementation of {@link
      * BackgroundTask#onStartTask(Context, TaskParameters, TaskFinishedCallback)} run before native