Modify ImageTransportSurfaceOverlayMac to allow pipelining

gpu/ipc/service/image_transport_surface_overlay_mac.mm

Index: gpu/ipc/service/image_transport_surface_overlay_mac.mm
diff --git a/gpu/ipc/service/image_transport_surface_overlay_mac.mm b/gpu/ipc/service/image_transport_surface_overlay_mac.mm
index 8f4e73002298232de83d01a6894e06c9d82fb39a..141184e173a77001a53b001c88cfa84407ccd4ba 100644
--- a/gpu/ipc/service/image_transport_surface_overlay_mac.mm
+++ b/gpu/ipc/service/image_transport_surface_overlay_mac.mm
@@ -157,10 +157,30 @@ gfx::SwapResult ImageTransportSurfaceOverlayMac::SwapBuffersInternal(
     const gfx::Rect& pixel_damage_rect) {
   TRACE_EVENT0("gpu", "ImageTransportSurfaceOverlayMac::SwapBuffersInternal");
 
-  // A glFlush is necessary to ensure correct content appears. A glFinish
-  // appears empirically to be the best way to get maximum performance when
-  // GPU bound.
-  {
+  // If supported, use GLFence to ensure that we haven't gotten more than one
+  // frame ahead of GL.
+  if (gl::GLFence::IsSupported()) {
+    // If we have gotten more than one frame ahead of GL, wait for the previous
+    // frame to complete.
+    if (previous_frame_fence_) {
+      TRACE_EVENT0("gpu", "ImageTransportSurfaceOverlayMac::ClientWait");
+      previous_frame_fence_->ClientWait();

[ccameron, 2016/06/01 21:26:37]

This will cause the CPU to spin up to 100% on Mac. See:
https://bugs.chromium.org/p/chromium/issues/detail?id=611805#c1

Maybe we can just do a
  whilee (!passed)
     Sleep(1msec)

1msec is a long time, but this only happens when we have >16msec of GPU work, so
it shouldn't hurt too much.

+      previous_frame_fence_ = nullptr;
+    }
+
+    // A glFlush is necessary to ensure correct content appears.
+    {
+      gl::ScopedSetGLToRealGLApi scoped_set_gl_api;
+      CheckGLErrors("Before flush");
+      glFlush();
+      CheckGLErrors("After flush");
+    }
+
+    // Create a new fence after flushing work from the current frame.
+    previous_frame_fence_.reset(gl::GLFence::Create());
+  } else {
+    // GLFence isn't supported - issue a glFinish on each frame to ensure
+    // there is backpressure from GL.
     gl::ScopedSetGLToRealGLApi scoped_set_gl_api;
     TRACE_EVENT0("gpu", "ImageTransportSurfaceOverlayMac::glFinish");
     CheckGLErrors("Before finish");