Move Pickle to base namespace.

Updates forward-declares and headers, but keeps a using statement in to avoid updating all users atomically.

TBR=jschuh for IPC stuff

Review URL: https://codereview.chromium.org/1149113006

Cr-Commit-Position: refs/heads/master@{#332352}
diff --git a/ipc/ipc_sync_message.h b/ipc/ipc_sync_message.h
index 2b00f03..44d0f4f 100644
--- a/ipc/ipc_sync_message.h
+++ b/ipc/ipc_sync_message.h
@@ -60,7 +60,7 @@
 
   // Given a reply message, returns an iterator to the beginning of the data
   // (i.e. skips over the synchronous specific data).
-  static PickleIterator GetDataIterator(const Message* msg);
+  static base::PickleIterator GetDataIterator(const Message* msg);
 
   // Given a synchronous message (or its reply), returns its id.
   static int GetMessageId(const Message& msg);
@@ -90,7 +90,7 @@
   // Derived classes need to implement this, using the given iterator (which
   // is skipped past the header for synchronous messages).
   virtual bool SerializeOutputParameters(const Message& msg,
-                                         PickleIterator iter) = 0;
+                                         base::PickleIterator iter) = 0;
 };
 
 // When sending a synchronous message, this structure contains an object