Support offscreen contexts which own their backing surface

content/renderer/renderer_blink_platform_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/renderer_blink_platform_impl.h"
 
 #include <memory>
 #include <utility>
 
 #include "base/command_line.h"
@@ skipping 996 matching lines @@
       std::string error_message(
           "OffscreenContext Creation failed, Shared context is lost");
       gl_info->errorMessage = WebString::fromUTF8(error_message);
       return nullptr;
     }
     share_context = share_provider_impl->context_provider();
   }
 
   bool is_software_rendering = gpu_channel_host->gpu_info().software_rendering;
 
-  // This is an offscreen context, which doesn't use the default frame buffer,
-  // so don't request any alpha, depth, stencil, antialiasing.
+  // This is an offscreen context. Generally it won't use the default
+  // frame buffer, in that case don't request any alpha, depth, stencil,
+  // antialiasing. But we do need those attributes for the "own
+  // offscreen surface" optimization which supports directly drawing
+  // to a custom surface backed frame buffer.
   gpu::gles2::ContextCreationAttribHelper attributes;
-  attributes.alpha_size = -1;
-  attributes.depth_size = 0;
-  attributes.stencil_size = 0;
-  attributes.samples = 0;
+  attributes.alpha_size = web_attributes.supportAlpha ? 8 : -1;
+  attributes.depth_size = web_attributes.supportDepth ? 24 : 0;
+  attributes.stencil_size = web_attributes.supportStencil ? 8 : 0;
+  attributes.samples = web_attributes.supportAntialias ? 4 : 0;
   attributes.sample_buffers = 0;
   attributes.bind_generates_resource = false;
   // Prefer discrete GPU for WebGL.
   attributes.gpu_preference = gl::PreferDiscreteGpu;
 
   attributes.fail_if_major_perf_caveat =
       web_attributes.failIfMajorPerformanceCaveat;
   DCHECK_GT(web_attributes.webGLVersion, 0u);
   DCHECK_LE(web_attributes.webGLVersion, 2u);
   if (web_attributes.webGLVersion == 2)
@@ skipping 249 matching lines @@
   return &trial_token_validator_;
 }
 
 void RendererBlinkPlatformImpl::workerContextCreated(
     const v8::Local<v8::Context>& worker) {
   GetContentClient()->renderer()->DidInitializeWorkerContextOnWorkerThread(
       worker);
 }
 
 }  // namespace content