[Android] Photo Picker: Fix crash when content resolver returns null.

According to the documentation, this can occur if the underlying
content provider returns null or crashes.

Bug: 1056911, 656015
Change-Id: I8171b0aa0b4b44093ff95660e47fee2473619b20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083021
Auto-Submit: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745960}

chrome/android/java/src/org/chromium/chrome/browser/photo_picker/FileEnumWorkerTask.java

@@ -160,6 +160,11 @@ class FileEnumWorkerTask extends AsyncTask<List<PickerBitmap>> {
         Cursor imageCursor =
                 mContentResolver.query(contentUri, selectColumns, whereClause, whereArgs, orderBy);
 
+        if (imageCursor == null) {
+            Log.e(TAG, "Content Resolver query() returned null");
+            return null;
+        }
+
         Log.i(TAG,
                 "Found " + imageCursor.getCount() + " media files, when requesting columns: "
                         + Arrays.toString(selectColumns) + ", with WHERE " + whereClause