Remove unused IPC::Message priority.

base/pickle.h

Index: base/pickle.h
diff --git a/base/pickle.h b/base/pickle.h
index cb9bab98965f141b5ea9bffe264ed5cc76ba77e2..29e9b228dd2c689c6da97ed534ba6593aa7c74b1 100644
--- a/base/pickle.h
+++ b/base/pickle.h
@@ -114,7 +114,7 @@ class BASE_EXPORT Pickle {
   // instead the data is merely referenced by this Pickle.  Only const methods
   // should be used on the Pickle when initialized this way.  The header
   // padding size is deduced from the data length.
-  Pickle(const char* data, int data_len);
+  Pickle(const char* data, size_t data_len);
 
   // Initializes a Pickle as a deep copy of another Pickle.
   Pickle(const Pickle& other);