replace flaky DCHECK in DrawingBuffer with early-out

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 695 matching lines @@
   } else {
     target = m_backColorBuffer->parameters.target;
     m_gl->GenMailboxCHROMIUM(mailbox.name);
     m_gl->ProduceTextureDirectCHROMIUM(m_backColorBuffer->textureId, target,
                                        mailbox.name);
     const GLuint64 fenceSync = m_gl->InsertFenceSyncCHROMIUM();
     m_gl->Flush();
     m_gl->GenSyncTokenCHROMIUM(fenceSync, produceSyncToken.GetData());
   }
 
-  DCHECK(produceSyncToken.HasData());
+  if (!produceSyncToken.HasData()) {
+    // This should only happen if the context has been lost.
+    return false;
+  }
+
   gl->WaitSyncTokenCHROMIUM(produceSyncToken.GetConstData());
   GLuint sourceTexture =
       gl->CreateAndConsumeTextureCHROMIUM(target, mailbox.name);
 
   GLboolean unpackPremultiplyAlphaNeeded = GL_FALSE;
   GLboolean unpackUnpremultiplyAlphaNeeded = GL_FALSE;
   if (m_wantAlphaChannel && m_premultipliedAlpha && !premultiplyAlpha)
     unpackUnpremultiplyAlphaNeeded = GL_TRUE;
   else if (m_wantAlphaChannel && !m_premultipliedAlpha && premultiplyAlpha)
     unpackPremultiplyAlphaNeeded = GL_TRUE;
@@ skipping 535 matching lines @@
   if (m_pixelUnpackBufferBindingDirty)
     client->DrawingBufferClientRestorePixelUnpackBufferBinding();
 }
 
 bool DrawingBuffer::shouldUseChromiumImage() {
   return RuntimeEnabledFeatures::webGLImageChromiumEnabled() &&
          m_chromiumImageUsage == AllowChromiumImage;
 }
 
 }  // namespace blink