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

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

Index: mojo/public/cpp/bindings/lib/fixed_buffer.h
diff --git a/mojo/public/cpp/bindings/lib/fixed_buffer.h b/mojo/public/cpp/bindings/lib/fixed_buffer.h
index 83eaf97067b37c38118488166fd566e862d01178..5e389f1fa5603360e2c1b4f5f2f327e3a3fb32d4 100644
--- a/mojo/public/cpp/bindings/lib/fixed_buffer.h
+++ b/mojo/public/cpp/bindings/lib/fixed_buffer.h
@@ -49,6 +49,8 @@ class FixedBuffer : public Buffer {
   // memory is zero-filled.
   void* Allocate(size_t num_bytes) override;
 
+  PickleBuffer* AsPickleBuffer() override;
+
  protected:
   char* ptr_;
   size_t cursor_;