cc: Cleanup - remove unused 'Webkit' forward declaration.

"namespace Webkit" does not exist.

R=danakj@chromium.org

Review URL: https://codereview.chromium.org/868353002

Cr-Commit-Position: refs/heads/master@{#313507}

cc/test/layer_tree_test.h

@@ -12,10 +12,6 @@
 #include "cc/trees/layer_tree_host_impl.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
-namespace Webkit {
-class WebGraphicsContext3D;
-}
-
 namespace cc {
 class FakeExternalBeginFrameSource;
 class FakeLayerTreeHostClient;