Enable creation of offscreen contexts which own their backing surface.

content/renderer/renderer_blink_platform_impl.cc

Index: content/renderer/renderer_blink_platform_impl.cc
diff --git a/content/renderer/renderer_blink_platform_impl.cc b/content/renderer/renderer_blink_platform_impl.cc
index 3d91912d563004b9334418bdb9400f8a6b6b2ad5..e22176578129b905f6e4f4168cc6fa8585db40ce 100644
--- a/content/renderer/renderer_blink_platform_impl.cc
+++ b/content/renderer/renderer_blink_platform_impl.cc
@@ -1020,17 +1020,29 @@ RendererBlinkPlatformImpl::createOffscreenGraphicsContext3DProvider(
   // This is an offscreen context, which doesn't use the default frame buffer,
   // so don't request any alpha, depth, stencil, antialiasing.
   gpu::gles2::ContextCreationAttribHelper attributes;
-  attributes.alpha_size = -1;
-  attributes.depth_size = 0;
-  attributes.stencil_size = 0;
-  attributes.samples = 0;
-  attributes.sample_buffers = 0;
+
+  if (web_attributes.supportOwnOffscreenSurface) {
+    attributes.own_offscreen_surface = true;
+    attributes.low_priority = true;
+    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;
+  } else {
+    attributes.alpha_size = -1;

[dcheng, 2017/01/09 23:14:57]

Nit: I think this is the default value already
(https://cs.chromium.org/chromium/src/gpu/command_buffer/common/gles2_cmd_util...),
so setting it seems unnecessary.

[klausw, 2017/01/09 23:33:24]

This matches the pre-patch logic, and in this case I think it's clearer to be
explicit. It doesn't care about alpha (leaving it implementation defined, cf.
comment in gpu_command_buffer_stub) while it specifically doesn't want
depth/stencil/antialiasing.

+    attributes.depth_size = 0;
+    attributes.stencil_size = 0;
+    attributes.samples = 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)