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

content/common/gpu/client/gl_helper.cc

Index: content/common/gpu/client/gl_helper.cc
diff --git a/content/common/gpu/client/gl_helper.cc b/content/common/gpu/client/gl_helper.cc
index 2ce57d9170115575ee0303764637d9aec07998b6..2972c32beb33d290d2b0060efe7446b035b75b7e 100644
--- a/content/common/gpu/client/gl_helper.cc
+++ b/content/common/gpu/client/gl_helper.cc
@@ -193,7 +193,8 @@ class GLHelper::CopyTextureToImpl :
           row_stride_bytes(row_stride_bytes_),
           pixels(pixels_),
           callback(callback_),
-          buffer(0) {
+          buffer(0),
+          query(0) {
     }
 
     gfx::Size size;
@@ -202,6 +203,7 @@ class GLHelper::CopyTextureToImpl :
     unsigned char* pixels;
     base::Callback<void(bool)> callback;
     GLuint buffer;
+    WebKit::WebGLId query;
   };
 
   // A readback pipeline that also converts the data to YUV before
@@ -380,12 +382,16 @@ void GLHelper::CopyTextureToImpl::ReadbackAsync(
                        NULL,
                        GL_STREAM_READ);
 
+  request->query = context_->createQueryEXT();
+  context_->beginQueryEXT(GL_ASYNC_READ_PIXELS_COMPLETED_CHROMIUM,
+                          request->query);
   context_->readPixels(0, 0, dst_size.width(), dst_size.height(),
                        GL_RGBA, GL_UNSIGNED_BYTE, NULL);
+  context_->endQueryEXT(GL_ASYNC_READ_PIXELS_COMPLETED_CHROMIUM);
   context_->bindBuffer(GL_PIXEL_PACK_TRANSFER_BUFFER_CHROMIUM, 0);
-  cc::SyncPointHelper::SignalSyncPoint(
+  cc::SyncPointHelper::SignalQuery(
       context_,
-      context_->insertSyncPoint(),
+      request->query,
       base::Bind(&CopyTextureToImpl::ReadbackDone, AsWeakPtr(), request));
 }
 
@@ -498,10 +504,15 @@ void GLHelper::CopyTextureToImpl::ReadbackDone(Request* request) {
 
 void GLHelper::CopyTextureToImpl::FinishRequest(Request* request,
                                                 bool result) {
+  TRACE_EVENT0("mirror", "GLHelper::CopyTextureToImpl::FinishRequest");
   DCHECK(request_queue_.front() == request);
   request_queue_.pop();
   request->callback.Run(result);
   ScopedFlush flush(context_);
+  if (request->query != 0) {
+    context_->deleteQueryEXT(request->query);
+    request->query = 0;
+  }
   if (request->buffer != 0) {
     context_->deleteBuffer(request->buffer);
     request->buffer = 0;