[mojo] Use base::Pickle for Message storage.

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..bfaea59f899bf755e227d260d70e0e3a73206aed 100644
--- a/mojo/public/cpp/bindings/lib/fixed_buffer.cc
+++ b/mojo/public/cpp/bindings/lib/fixed_buffer.cc
@@ -38,6 +38,8 @@ void* FixedBuffer::Allocate(size_t delta) {
   return result;
 }
 
+PickleBuffer* FixedBuffer::AsPickleBuffer() { return nullptr; }
+
 FixedBufferForTesting::FixedBufferForTesting(size_t size) {
   size_ = internal::Align(size);
   // Use calloc here to ensure all message memory is zero'd out.