Use WebCompositor only when --enable-threaded-compositing.

content/renderer/render_widget.cc

Index: content/renderer/render_widget.cc
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index 960fa49933998665f336e8b1172c1682d9403995..d5a27e1362007f2873bd169919d593a26fe6b4a2 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -925,8 +925,10 @@ void RenderWidget::didScrollRect(int dx, int dy, const WebRect& clip_rect) {
 void RenderWidget::didActivateCompositor(int compositor_identifier) {
   TRACE_EVENT0("gpu", "RenderWidget::didActivateCompositor");
 
-  RenderThreadImpl::current()->compositor_thread()->
-      AddCompositor(routing_id_, compositor_identifier);
+  CompositorThread* compositor_thread =
+      RenderThreadImpl::current()->compositor_thread();
+  if (compositor_thread)

[jamesr, 2011/10/20 22:51:52]

in webkit we normally fold the assignment into the if() to scope the local. Do
we do this in chrome?

[darin (slow to review), 2011/10/21 05:03:35]

I don't think there is a strong convention.  It'd probably lead to some link
breaking here, which might be worse :)

+    compositor_thread->AddCompositor(routing_id_, compositor_identifier);
 
   is_accelerated_compositing_active_ = true;
   Send(new ViewHostMsg_DidActivateAcceleratedCompositing(