WebVR: clear screen after submit if preserveDrawingBuffer is false.

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

Index: third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
index 833553d48acd4cc118a29b844420ec3f01fc98ee..ceada6f99d70a5b970eecfcbca4b2276caba176c 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
+++ b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
@@ -1517,6 +1517,11 @@ WebGLRenderingContextBase::clearIfComposited(GLbitfield mask) {
   return combinedClear ? CombinedClear : JustClear;
 }
 
+void WebGLRenderingContextBase::markCompositedAndClearBackbufferIfNeeded() {
+  markLayerComposited();
+  clearIfComposited();
+}
+
 void WebGLRenderingContextBase::restoreScissorEnabled() {
   if (isContextLost())
     return;