DNT should return 'null' if there is no client.

BUG=347020
TEST=https://codereview.chromium.org/208243016/

Review URL: https://codereview.chromium.org/208333018

git-svn-id: svn://svn.chromium.org/blink/trunk@170046 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/donottrack/NavigatorDoNotTrack.cpp

@@ -68,7 +68,9 @@ String NavigatorDoNotTrack::doNotTrack(Navigator& navigator)
 
 String NavigatorDoNotTrack::doNotTrack()
 {
-    return frame() ? frame()->loader().client()->doNotTrackValue() : String();
+    if (!frame() || !frame()->loader().client())
+        return String();
+    return frame()->loader().client()->doNotTrackValue();
 }
 
 } // namespace WebCore