Bug 1257667 - Try to catch stale cursor usage using LeakCanary r=sebastian

MozReview-Commit-ID: Ga5lPgwpIxp

mobile/android/base/java/org/mozilla/gecko/home/SimpleCursorLoader.java

@@ -23,6 +23,8 @@ import android.content.Context;
 import android.database.Cursor;
 import android.support.v4.content.AsyncTaskLoader;
 
+import org.mozilla.gecko.GeckoApplication;
+
 /**
  * A copy of the framework's {@link android.content.CursorLoader} that
  * instead allows the caller to load the Cursor themselves via the abstract
@@ -87,6 +89,10 @@ abstract class SimpleCursorLoader extends AsyncTaskLoader<Cursor> {
         }
 
         if (oldCursor != null && oldCursor != cursor && !oldCursor.isClosed()) {
+            // Trying to read from the closed cursor will cause crashes, hence we should make
+            // sure that no adapters/LoaderCallbacks are holding onto this cursor.
+            GeckoApplication.getRefWatcher(getContext()).watch(oldCursor);
+
             oldCursor.close();
         }
     }