Enable creation of offscreen contexts which own their backing surface.

gpu/ipc/service/gpu_command_buffer_stub.cc

Index: gpu/ipc/service/gpu_command_buffer_stub.cc
diff --git a/gpu/ipc/service/gpu_command_buffer_stub.cc b/gpu/ipc/service/gpu_command_buffer_stub.cc
index 3f41a30555095398eacb3ddab053551bfa5bfb3d..17b4b1162cc5455fff0badeb4bbf62ed2a7b34fd 100644
--- a/gpu/ipc/service/gpu_command_buffer_stub.cc
+++ b/gpu/ipc/service/gpu_command_buffer_stub.cc
@@ -623,7 +623,29 @@ bool GpuCommandBufferStub::Initialize(
   decoder_->set_engine(executor_.get());
 
   if (offscreen) {
-    surface_ = default_surface;
+    if (init_params.attribs.own_offscreen_surface) {
+      DVLOG(1) << __FUNCTION__ << "Hit the own_offscreen_surface path";
+      use_virtualized_gl_context_ = false;

[jbauman, 2017/01/06 03:09:08]

We probably need to actually fail context creation if
use_virtualized_gl_context_ is true in this case. Otherwise we'll hit a lot of
driver bugs (which is why we have those workarounds in the first place).

[klausw, 2017/01/06 06:13:00]

I don't have a strong opinion about this, but the pre-existing code right above
this (line 608) just sets use_virtualized_gl_context_ to 
false if the onscreen buffer's format is incompatible with the default
surface, without raising an error if it was originally set, and that's
a fairly similar scenario.

What happens when it returns false here? Does WebGL content simply fail
to render, and does it report this back to the user in some way?

+      // Currently, we always get alpha channe by default.
+      // TODO(klausw): add way to disable alpha?
+      if (init_params.attribs.depth_size > 0) {
+        surface_format.SetDepthBits(init_params.attribs.depth_size);
+      }
+      if (init_params.attribs.samples > 0) {
+        surface_format.SetSamples(init_params.attribs.samples);
+      }
+      if (init_params.attribs.stencil_size > 0) {
+        surface_format.SetStencilBits(init_params.attribs.stencil_size);
+      }
+      surface_ = gl::init::CreateOffscreenGLSurfaceWithFormat(
+          gfx::Size(), surface_format);
+      if (!surface_) {
+        DLOG(ERROR) << "Failed to create surface.";
+        return false;
+      }
+    } else {
+      surface_ = default_surface;
+    }
   } else {
     surface_ = ImageTransportSurface::CreateNativeSurface(
         AsWeakPtr(), surface_handle_, surface_format);