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

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index d9729d33ce41e7e517e27e9928a9f1d8577789dd..80d2ea63b432c05743a39b9370c3142ac2a3737d 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -296,7 +296,8 @@ void GLRenderer::ClearFramebuffer(DrawingFrame* frame) {
 }
 
 void GLRenderer::BeginDrawingFrame(DrawingFrame* frame) {
-  // FIXME: Remove this once backbuffer is automatically recreated on first use
+  // TODO(hubbe): Remove this once backbuffer is automatically recreated
+  // on first use.
   EnsureBackbuffer();
 
   if (client_->DeviceViewport().IsEmpty())
@@ -586,22 +587,22 @@ scoped_ptr<ScopedResource> GLRenderer::DrawBackgroundFilters(
   //
   // Pixel copies in this algorithm occur at steps 2, 3, 4, and 5.
 
-  // FIXME: When this algorithm changes, update
+  // TODO(hubbe): When this algorithm changes, update
   // LayerTreeHost::PrioritizeTextures() accordingly.
 
   FilterOperations filters =
       RenderSurfaceFilters::Optimize(quad->background_filters);
   DCHECK(!filters.IsEmpty());
 
-  // FIXME: We only allow background filters on an opaque render surface because
-  // other surfaces may contain translucent pixels, and the contents behind
-  // those translucent pixels wouldn't have the filter applied.
+  // TODO(hubbe): We only allow background filters on an opaque render surface
+  // because other surfaces may contain translucent pixels, and the contents
+  // behind those translucent pixels wouldn't have the filter applied.
   if (frame->current_render_pass->has_transparent_background)
     return scoped_ptr<ScopedResource>();
   DCHECK(!frame->current_texture);
 
-  // FIXME: Do a single readback for both the surface and replica and cache the
-  // filtered results (once filter textures are not reused).
+  // TODO(hubbe): Do a single readback for both the surface and replica and
+  // cache the filtered results (once filter textures are not reused).
   gfx::Rect window_rect = gfx::ToEnclosingRect(MathUtil::MapClippedRect(
       contents_device_transform, SharedGeometryQuad().BoundingBox()));
 
@@ -722,8 +723,8 @@ void GLRenderer::DrawRenderPassQuad(DrawingFrame* frame,
       SetBlendEnabled(true);
   }
 
-  // FIXME: Cache this value so that we don't have to do it for both the surface
-  // and its replica.  Apply filters to the contents texture.
+  // TODO(hubbe): Cache this value so that we don't have to do it for both the
+  // surface and its replica.  Apply filters to the contents texture.
   SkBitmap filter_bitmap;
   SkScalar color_matrix[20];
   bool use_color_matrix = false;
@@ -794,8 +795,9 @@ void GLRenderer::DrawRenderPassQuad(DrawingFrame* frame,
     mask_texture_id = mask_resource_lock->texture_id();
   }
 
-  // FIXME: use the background_texture and blend the background in with this
-  // draw instead of having a separate copy of the background texture.
+  // TODO(hubbe): use the background_texture and blend the background
+  // in with this draw instead of having a separate copy of the background
+  // texture.
 
   scoped_ptr<ResourceProvider::ScopedReadLockGL> contents_resource_lock;
   if (filter_bitmap.getTexture()) {
@@ -2310,6 +2312,14 @@ void GLRenderer::DoGetFramebufferPixels(
                                      NULL,
                                      GL_STREAM_READ));
 
+  WebKit::WebGLId query = 0;
+  if (is_async) {
+    query = context_->createQueryEXT();
+    GLC(context_, context_->beginQueryEXT(
+        GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM,
+        query));
+  }
+
   GLC(context_,
       context_->readPixels(window_rect.x(),
                            window_rect.y(),
@@ -2335,6 +2345,7 @@ void GLRenderer::DoGetFramebufferPixels(
                  base::Unretained(this),
                  cleanup_callback,
                  buffer,
+                 query,
                  dest_pixels,
                  window_rect.size());
   // Save the finished_callback so it can be cancelled.
@@ -2345,10 +2356,11 @@ void GLRenderer::DoGetFramebufferPixels(
   pending_async_read_pixels_.front()->buffer = buffer;
 
   if (is_async) {
-    unsigned sync_point = context_->insertSyncPoint();
-    SyncPointHelper::SignalSyncPoint(
+    GLC(context_, context_->endQueryEXT(
+        GL_ASYNC_PIXEL_TRANSFERS_COMPLETED_CHROMIUM));
+    SyncPointHelper::SignalQuery(
         context_,
-        sync_point,
+        query,
         finished_callback);
   } else {
     resource_provider_->Finish();
@@ -2361,10 +2373,15 @@ void GLRenderer::DoGetFramebufferPixels(
 void GLRenderer::FinishedReadback(
     const AsyncGetFramebufferPixelsCleanupCallback& cleanup_callback,
     unsigned source_buffer,
+    unsigned query,
     uint8* dest_pixels,
     gfx::Size size) {
   DCHECK(!pending_async_read_pixels_.empty());
 
+  if (query != 0) {
+    GLC(context_, context_->deleteQueryEXT(query));
+  }
+
   PendingAsyncReadPixels* current_read = pending_async_read_pixels_.back();
   // Make sure we service the readbacks in order.
   DCHECK_EQ(source_buffer, current_read->buffer);