Add a null check for the target node in page_click_tracker.

Events arising from clicks on instance elements in SVG <use> trees may have their
target changed when the instance tree is rebuilt during event processing. In this
case the toNode() call will return NULL for the target.

BUG=171363

Review URL: https://chromiumcodereview.appspot.com/12248003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@182049 0039d316-1c4b-4281-b951-d872f2087c98

chrome/renderer/page_click_tracker.cc

@@ -132,6 +132,10 @@ void PageClickTracker::handleEvent(const WebDOMEvent& event) {
   // We'll get a notification once the mouse event has been processed
   // (DidHandleMouseEvent), we'll notify the listener at that point.
   WebNode node = mouse_event.target();
+  if (node.isNull())
+    // Node may be null if the target was an SVG instance element from a <use>
+    // tree and the tree has been rebuilt due to an earlier event.
+    return;
 
   HandleTextFieldMaybeLosingFocus(node);