gpu: Add sync token dependencies to flush metadata.

ppapi/proxy/ppapi_command_buffer_proxy.cc

Index: ppapi/proxy/ppapi_command_buffer_proxy.cc
diff --git a/ppapi/proxy/ppapi_command_buffer_proxy.cc b/ppapi/proxy/ppapi_command_buffer_proxy.cc
index 8a5d0622a707e83a0a549fe310aae6717a62b88f..2ba3e8114a29fe7be245da8aa498d42f33c5bc05 100644
--- a/ppapi/proxy/ppapi_command_buffer_proxy.cc
+++ b/ppapi/proxy/ppapi_command_buffer_proxy.cc
@@ -214,8 +214,10 @@ void PpapiCommandBufferProxy::SignalSyncToken(const gpu::SyncToken& sync_token,
   NOTIMPLEMENTED();
 }
 
+void PpapiCommandBufferProxy::WaitSyncToken(const gpu::SyncToken& sync_token) {}
+
 bool PpapiCommandBufferProxy::CanWaitUnverifiedSyncToken(
-    const gpu::SyncToken* sync_token) {
+    const gpu::SyncToken& sync_token) {
   return false;
 }