Fix lost context race with fullscreen pepper widget.

content/renderer/render_widget_fullscreen_pepper.cc

Index: content/renderer/render_widget_fullscreen_pepper.cc
diff --git a/content/renderer/render_widget_fullscreen_pepper.cc b/content/renderer/render_widget_fullscreen_pepper.cc
index b8d144da46450341d419cad2ddbe2a4f4438ca5a..923cb487c66420caf1f46337ecb4ff4bbac75596 100644
--- a/content/renderer/render_widget_fullscreen_pepper.cc
+++ b/content/renderer/render_widget_fullscreen_pepper.cc
@@ -462,10 +462,10 @@ bool RenderWidgetFullscreenPepper::InitContext() {
 }
 
 bool RenderWidgetFullscreenPepper::CheckCompositing() {
-  bool compositing = webwidget_->isAcceleratedCompositingActive();
-  if (compositing != is_accelerated_compositing_active_) {
+  bool compositing =
+      webwidget_ && webwidget_->isAcceleratedCompositingActive();
+  if (compositing != is_accelerated_compositing_active_)
     didActivateAcceleratedCompositing(compositing);
-  }
   return compositing;
 }
 
@@ -489,6 +489,7 @@ void RenderWidgetFullscreenPepper::OnLostContext(
   program_ = 0;
   buffer_ = 0;
   OnSwapBuffersAborted();
+  CheckCompositing();
 }
 
 void RenderWidgetFullscreenPepper::OnSwapBuffersCompleteByRendererGLContext() {