Implementation of preserveDrawingBuffer:Discard in software

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

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 909e548229317c4bcb33c808427e72162135df73..9dfb416e2b34ab7d6a3d8572306396f21b0cb405 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -324,6 +324,11 @@ bool DrawingBuffer::finishPrepareTextureMailboxSoftware(
                         WTF::passed(std::move(bitmap)), m_size);
   *outReleaseCallback =
       cc::SingleReleaseCallback::Create(convertToBaseCallback(std::move(func)));
+
+  if (m_preserveDrawingBuffer == Discard) {
+    setBufferClearNeeded(true);
+  }
+
   return true;
 }