GPU: Add trace for real SwapBuffers.

ui/gl/gl_surface_nsview.mm

Index: ui/gl/gl_surface_nsview.mm
diff --git a/ui/gl/gl_surface_nsview.mm b/ui/gl/gl_surface_nsview.mm
index ded07197108e0df232bf15fc784311d84505df88..9818dbfb1c57317aa66bcf66abcb597e50c442b3 100644
--- a/ui/gl/gl_surface_nsview.mm
+++ b/ui/gl/gl_surface_nsview.mm
@@ -7,6 +7,7 @@
 #import <AppKit/NSOpenGL.h>
 #import <AppKit/NSView.h>
 
+#include "base/debug/trace_event.h"
 #include "ui/gl/gl_context_nsview.h"
 
 namespace gfx {
@@ -27,6 +28,9 @@ bool GLSurfaceNSView::IsOffscreen() {
 }
 
 bool GLSurfaceNSView::SwapBuffers() {
+  TRACE_EVENT2("gpu", "GLSurfaceNSView:RealSwapBuffers",
+      "width", GetSize().width(),
+      "height", GetSize().height());
   context_->FlushBuffer();
   return true;
 }