[mojo] Add Mojo bindings support for IPC::ParamTraits

mojo/public/cpp/bindings/lib/fixed_buffer.cc

Index: mojo/public/cpp/bindings/lib/fixed_buffer.cc
diff --git a/mojo/public/cpp/bindings/lib/fixed_buffer.cc b/mojo/public/cpp/bindings/lib/fixed_buffer.cc
index c81fc6e8ed8bf70d7e6b325e6de8f8b994007c3d..abcd3ebfce4c7a22fd3b15183b654c3dc2a5057a 100644
--- a/mojo/public/cpp/bindings/lib/fixed_buffer.cc
+++ b/mojo/public/cpp/bindings/lib/fixed_buffer.cc
@@ -24,6 +24,8 @@ void FixedBuffer::Initialize(void* memory, size_t size) {
   size_ = size;
 }
 
+PickleBuffer* FixedBuffer::AsPickleBuffer() { return nullptr; }
+
 void* FixedBuffer::Allocate(size_t delta) {
   delta = internal::Align(delta);