Call OnSwapBuffersComplete at the end of each frame

cc/output/output_surface.cc

Index: cc/output/output_surface.cc
diff --git a/cc/output/output_surface.cc b/cc/output/output_surface.cc
index 1be41892a648a16d3ab2f6496bae61d0cc0d1f23..dc34eab4b9c8d05280fff9a0ad7f7342bbfdc66b 100644
--- a/cc/output/output_surface.cc
+++ b/cc/output/output_surface.cc
@@ -136,10 +136,11 @@ void OutputSurface::BindFramebuffer() {
 }
 
 void OutputSurface::SwapBuffers(const ui::LatencyInfo& latency_info) {
-  DCHECK(context3d_);
-  // Note that currently this has the same effect as SwapBuffers; we should
-  // consider exposing a different entry point on WebGraphicsContext3D.
-  context3d_->prepareTexture();
+  if (context3d_) {
+    // Note that currently this has the same effect as SwapBuffers; we should
+    // consider exposing a different entry point on WebGraphicsContext3D.
+    context3d_->prepareTexture();
+  }
 }
 
 void OutputSurface::PostSubBuffer(gfx::Rect rect,