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

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

Index: mojo/public/cpp/bindings/lib/buffer.h
diff --git a/mojo/public/cpp/bindings/lib/buffer.h b/mojo/public/cpp/bindings/lib/buffer.h
index c3b570e7767d5aee8a2e3593fa9a2d849d3e4179..98bbce0e6aef705c0f638cf26796c168c5d506ca 100644
--- a/mojo/public/cpp/bindings/lib/buffer.h
+++ b/mojo/public/cpp/bindings/lib/buffer.h
@@ -10,12 +10,18 @@
 namespace mojo {
 namespace internal {
 
+class PickleBuffer;
+
 // Buffer provides a way to allocate memory. Allocations are 8-byte aligned and
 // zero-initialized. Allocations remain valid for the lifetime of the Buffer.
 class Buffer {
  public:
   virtual ~Buffer() {}
   virtual void* Allocate(size_t num_bytes) = 0;
+
+  // TODO(rockot): Remove this. It's a hack to get a PickleBuffer in
+  // Serialize_ calls without having to update every call site.
+  virtual PickleBuffer* AsPickleBuffer() = 0;
 };
 
 }  // namespace internal