Perform glReadPixels with PBOs in the gpu, if PBOs are available.

gpu/command_buffer/service/feature_info.cc

Index: gpu/command_buffer/service/feature_info.cc
diff --git a/gpu/command_buffer/service/feature_info.cc b/gpu/command_buffer/service/feature_info.cc
index 5d9584827d4fe4d36f1259d9877707d6c7f99fed..396311cffb1b6690d7c84ff0a2052ebc885206e1 100644
--- a/gpu/command_buffer/service/feature_info.cc
+++ b/gpu/command_buffer/service/feature_info.cc
@@ -117,7 +117,8 @@ FeatureInfo::FeatureFlags::FeatureFlags()
       enable_shader_name_hashing(false),
       enable_samplers(false),
       ext_draw_buffers(false),
-      ext_frag_depth(false) {
+      ext_frag_depth(false),
+      use_async_readpixels(false) {
 }
 
 FeatureInfo::Workarounds::Workarounds() :
@@ -623,6 +624,16 @@ void FeatureInfo::AddFeatures(const CommandLine& command_line) {
     feature_flags_.ext_frag_depth = true;
   }
 
+  bool ui_gl_fence_works =
+      extensions.Contains("GL_NV_fence") ||
+      extensions.Contains("GL_ARB_sync");
+
+  if (ui_gl_fence_works &&
+      extensions.Contains("GL_ARB_pixel_buffer_object") &&
+      !workarounds_.disable_async_readpixels) {
+    feature_flags_.use_async_readpixels = true;
+  }
+
   if (!disallowed_features_.swap_buffer_complete_callback)
     AddExtensionString("GL_CHROMIUM_swapbuffers_complete_callback");