WebVR: clear screen after submit if preserveDrawingBuffer is false.

third_party/WebKit/Source/modules/vr/VRDisplay.cpp

Index: third_party/WebKit/Source/modules/vr/VRDisplay.cpp
diff --git a/third_party/WebKit/Source/modules/vr/VRDisplay.cpp b/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
index 2fb51eb7a22bc52a12f9de73a174f70efa99d492..6f13f70e67a81d7f10850cbfc01f761ef2435a06 100644
--- a/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
+++ b/third_party/WebKit/Source/modules/vr/VRDisplay.cpp
@@ -600,6 +600,11 @@ void VRDisplay::submitFrame() {
       m_vrFrameId, gpu::MailboxHolder(staticImage->mailbox(),
                                       staticImage->syncToken(), GL_TEXTURE_2D));
 
+  // If preserveDrawingBuffer is false, must clear now. Normally this
+  // happens as part of compositing, but that's not active while
+  // presenting, so run the responsible code directly.
+  m_renderingContext->markCompositedAndClearBackbufferIfNeeded();
+
   // If we're not deferring the wait for transferring the mailbox,
   // we need to wait for it now to prevent the image going out of
   // scope before its mailbox is retrieved.