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 339af699cf3917c0519e1e23228f5ab38b7ae9fb..7adf1ff81b0385232a6ce3e9328879cecc10730e 100644
--- a/content/renderer/renderer_blink_platform_impl.cc
+++ b/content/renderer/renderer_blink_platform_impl.cc
@@ -1025,20 +1025,31 @@ RendererBlinkPlatformImpl::createOffscreenGraphicsContext3DProvider(
 
   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.
   gpu::gles2::ContextCreationAttribHelper attributes;
-  attributes.alpha_size = -1;
-  attributes.depth_size = 0;
-  attributes.stencil_size = 0;
-  attributes.samples = 0;
-  attributes.sample_buffers = 0;
+
+  attributes.own_offscreen_surface = web_attributes.ownOffscreenSurface;
+  if (web_attributes.ownOffscreenSurface) {
+    attributes.alpha_size = web_attributes.alpha ? 8 : -1;
+    attributes.depth_size = web_attributes.depth ? 24 : 0;
+    attributes.stencil_size = web_attributes.stencil ? 8 : 0;
+    attributes.samples = web_attributes.antialias ? 4 : 0;
+    attributes.sample_buffers = web_attributes.antialias ? 1 : 0;
+  } else {
+    // This is an offscreen context, which doesn't use the default frame buffer,
+    // so don't request any alpha, depth, stencil, antialiasing.
+    attributes.alpha_size = -1;
+    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)