Make CommandBuffer::Flush asynchronous, and add CommandBuffer::FlushSync with former semantics.

gpu/pgl/command_buffer_pepper.cc

Index: gpu/pgl/command_buffer_pepper.cc
diff --git a/gpu/pgl/command_buffer_pepper.cc b/gpu/pgl/command_buffer_pepper.cc
index 0025803de7879fea27743d52be5cc16156578bed..db724744ffc1fc35bc0c77a0488d1606ea4b892c 100644
--- a/gpu/pgl/command_buffer_pepper.cc
+++ b/gpu/pgl/command_buffer_pepper.cc
@@ -83,7 +83,11 @@ CommandBuffer::State CommandBufferPepper::GetState() {
 #endif  // ENABLE_NEW_NPDEVICE_API
 }
 
-CommandBuffer::State CommandBufferPepper::Flush(int32 put_offset) {
+void CommandBufferPepper::Flush(int32 put_offset) {
+  FlushSync(put_offset);
+}
+
+CommandBuffer::State CommandBufferPepper::FlushSync(int32 put_offset) {
 #if defined(ENABLE_NEW_NPDEVICE_API)
   int32 input_attribs[] = {
     NP3DAttrib_PutOffset, put_offset,