Gallery: Fix the slide show mode.

Recent change (crrev.com/398263002) broke the slide show mode and we cannot
enter the slide show mode from the mosaic view due. The CL adds null check to
fix it.

BUG=245926
TEST=manually

Review URL: https://codereview.chromium.org/403733002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284042 0039d316-1c4b-4281-b951-d872f2087c98

ui/file_manager/gallery/js/image_editor/image_view.js

@@ -129,10 +129,12 @@ ImageView.prototype.draw = function() {
  * change or offset change) with animation.
  */
 ImageView.prototype.applyViewportChange = function() {
-  this.setTransform(
-      this.screenImage_,
-      new ImageView.Effect.None(),
-      ImageView.Effect.DEFAULT_DURATION);
+  if (this.screenImage_) {
+    this.setTransform(
+        this.screenImage_,
+        new ImageView.Effect.None(),
+        ImageView.Effect.DEFAULT_DURATION);
+  }
 };
 
 /**