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..314db23f83e7bdc9fda242f12832dc938da8c39d 100644
--- a/third_party/WebKit/public/platform/Platform.h
+++ b/third_party/WebKit/public/platform/Platform.h
@@ -460,6 +460,12 @@ class BLINK_PLATFORM_EXPORT Platform {
   struct ContextAttributes {
     bool failIfMajorPerformanceCaveat = false;
     unsigned webGLVersion = 0;
+    bool supportOwnOffscreenSurface = false;
+    // The remaining values are only used if supportOwnOffscreenSurface is true.
+    bool supportAlpha = true;
+    bool supportDepth = false;
+    bool supportAntialias = true;
+    bool supportStencil = false;
   };
   struct GraphicsInfo {
     unsigned vendorId = 0;