Add a missing null check in SliderThumbElement::stopDragging()

LayoutTests/media/audio-delete-while-slider-thumb-clicked.html

Index: LayoutTests/media/audio-delete-while-slider-thumb-clicked.html
diff --git a/LayoutTests/media/audio-delete-while-slider-thumb-clicked.html b/LayoutTests/media/audio-delete-while-slider-thumb-clicked.html
index 052dadf5fb3193809f73e0e96ab667f229e23143..59959c8067ecdbbefd7e496ec49bc68dbe43edf9 100644
--- a/LayoutTests/media/audio-delete-while-slider-thumb-clicked.html
+++ b/LayoutTests/media/audio-delete-while-slider-thumb-clicked.html
@@ -15,9 +15,12 @@
                 var console = document.getElementById('console');
                 console.innerHTML = console.innerHTML + msg + "<br>";
             }
-            
+
             function buttonClick()
             {
+                if (window.GCController)

[acolwell GONE FROM CHROMIUM, 2014/03/25 22:28:57]

nit: I think you should move this into deleteAudio() since this clearly isn't a
button click and it's a little confusing to have it here. Adding a comment about
why you GC in some testing environments and force a click in others would be
nice.

[philipj_slow, 2014/03/26 08:15:32]

Ahem... this code was copied from adopt-node-crash.html, but I messed up by
copying the return as well, fortunately you didn't miss it!

I didn't put it in deleteAudio() because that's run in an event listener for the
audio element, and the audio element cannot be collected at that point, at least
not in my testing.

I'll fix this by copying all of forceGC() instead, which should make things
clearer.

+                    return GCController.collect();
+
                 if (document.getElementById('audio'))
                     log("<br>FAIL: audio element not deleted!!");
 
@@ -31,7 +34,7 @@
 
                 testRunner.notifyDone();
             }
-            
+
             function deleteAudio()
             {
                 var audio = document.getElementById('audio');
@@ -43,7 +46,7 @@
 
                 setTimeout(buttonClick, 10);
             }
-            
+
             function drag()
             {
                 if (!window.testRunner)