Support offscreen contexts which own their backing surface

third_party/WebKit/public/platform/Platform.h

Index: third_party/WebKit/public/platform/Platform.h
diff --git a/third_party/WebKit/public/platform/Platform.h b/third_party/WebKit/public/platform/Platform.h
index 5835825aaaabaf8c720e15860bf1ffbb7764114c..9d5ae85fbf9e8c7d1d18238f110f96c430cbbe93 100644
--- a/third_party/WebKit/public/platform/Platform.h
+++ b/third_party/WebKit/public/platform/Platform.h
@@ -460,6 +460,13 @@ class BLINK_PLATFORM_EXPORT Platform {
   struct ContextAttributes {
     bool failIfMajorPerformanceCaveat = false;
     unsigned webGLVersion = 0;
+    // Offscreen contexts usually share a surface for the default frame buffer
+    // since they aren't rendering to it. Setting any of the following
+    // attributes causes creation of a custom surface owned by the context.
+    bool supportAlpha = false;
+    bool supportDepth = false;
+    bool supportAntialias = false;
+    bool supportStencil = false;
   };
   struct GraphicsInfo {
     unsigned vendorId = 0;