Use glDiscardFramebuffer to save memory bandwidth

Source/platform/graphics/gpu/DrawingBufferTest.cpp

Index: Source/platform/graphics/gpu/DrawingBufferTest.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBufferTest.cpp b/Source/platform/graphics/gpu/DrawingBufferTest.cpp
index fdaadf7835ad135463e99c532689bf7d56afb50c..d5e8cf3becb386b3c2b6aa557b1f6350f92b7ea5 100644
--- a/Source/platform/graphics/gpu/DrawingBufferTest.cpp
+++ b/Source/platform/graphics/gpu/DrawingBufferTest.cpp
@@ -190,7 +190,7 @@ public:
         PreserveDrawingBuffer preserve,
         PassRefPtr<ContextEvictionManager> contextEvictionManager)
         : DrawingBuffer(context, extensionsUtil, false /* multisampleExtensionSupported */,
-            false /* packedDepthStencilExtensionSupported */, preserve, WebGraphicsContext3D::Attributes(), contextEvictionManager)
+            false /* packedDepthStencilExtensionSupported */, false /* discardFramebufferSupported */, preserve, WebGraphicsContext3D::Attributes(), contextEvictionManager)
         , m_live(0)
     { }