Support offscreen contexts which own their backing surface

third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp

Index: third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp b/third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp
index bf24b48cc493db93e2ab13e909380e1816151277..8c07036eba54d51150e64e898454332e50f55c89 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp
+++ b/third_party/WebKit/Source/modules/webgl/WebGLContextAttributeHelpers.cpp
@@ -23,10 +23,19 @@ WebGLContextAttributes toWebGLContextAttributes(
 
 Platform::ContextAttributes toPlatformContextAttributes(
     const CanvasContextCreationAttributes& attrs,
-    unsigned webGLVersion) {
+    unsigned webGLVersion,
+    bool supportOwnOffscreenSurface) {
   Platform::ContextAttributes result;
   result.failIfMajorPerformanceCaveat = attrs.failIfMajorPerformanceCaveat();
   result.webGLVersion = webGLVersion;
+  if (supportOwnOffscreenSurface) {
+    // Only ask for alpha/depth/stencil/antialias if we may be using the default
+    // framebuffer. They are not needed for standard offscreen rendering.
+    result.supportAlpha = attrs.alpha();
+    result.supportDepth = attrs.depth();
+    result.supportStencil = attrs.stencil();
+    result.supportAntialias = attrs.antialias();
+  }
   return result;
 }