WebVR: clear screen after submit if preserveDrawingBuffer is false.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/vr/VRDisplay.h"
 
 #include "core/css/StylePropertySet.h"
 #include "core/dom/DOMException.h"
 #include "core/dom/FrameRequestCallback.h"
 #include "core/dom/ScriptedAnimationController.h"
@@ skipping 582 matching lines @@
       }
     }
   }
 
   m_pendingPreviousFrameRender = true;
   m_pendingSubmitFrame = true;
   m_display->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.
   if (!waitForPreviousTransferToFinish) {
     TRACE_EVENT0("gpu", "waitForCurrentTransferToFinish");
     while (m_pendingSubmitFrame) {
       if (!m_submit_frame_client_binding.WaitForIncomingMethodCall()) {
         DLOG(ERROR) << "Failed to receive SubmitFrame response";
         break;
       }
@@ skipping 191 matching lines @@
   visitor->trace(m_stageParameters);
   visitor->trace(m_eyeParametersLeft);
   visitor->trace(m_eyeParametersRight);
   visitor->trace(m_layer);
   visitor->trace(m_renderingContext);
   visitor->trace(m_scriptedAnimationController);
   visitor->trace(m_pendingPresentResolvers);
 }
 
 }  // namespace blink