Convert BeginReadData...() to use the new user pointer handling (see r285350).

mojo/system/data_pipe.h

Index: mojo/system/data_pipe.h
diff --git a/mojo/system/data_pipe.h b/mojo/system/data_pipe.h
index cd7c96d8ed64fe339bdc5b120b146647a3b18109..60f77bcd27146337a216334e810fc3bfeaee4cef 100644
--- a/mojo/system/data_pipe.h
+++ b/mojo/system/data_pipe.h
@@ -77,9 +77,8 @@ class MOJO_SYSTEM_IMPL_EXPORT DataPipe :
   MojoResult ConsumerDiscardData(uint32_t* num_bytes,
                                  bool all_or_none);
   MojoResult ConsumerQueryData(uint32_t* num_bytes);
-  // This does not validate its arguments.
-  MojoResult ConsumerBeginReadData(const void** buffer,
-                                   uint32_t* buffer_num_bytes,
+  MojoResult ConsumerBeginReadData(UserPointer<const void*> buffer,
+                                   UserPointer<uint32_t> buffer_num_bytes,
                                    bool all_or_none);
   MojoResult ConsumerEndReadData(uint32_t num_bytes_read);
   MojoResult ConsumerAddWaiter(Waiter* waiter,
@@ -119,9 +118,10 @@ class MOJO_SYSTEM_IMPL_EXPORT DataPipe :
                                                    bool all_or_none) = 0;
   // |*num_bytes| will be a nonzero multiple of |element_num_bytes_|.
   virtual MojoResult ConsumerQueryDataImplNoLock(uint32_t* num_bytes) = 0;
-  virtual MojoResult ConsumerBeginReadDataImplNoLock(const void** buffer,
-                                                     uint32_t* buffer_num_bytes,
-                                                     bool all_or_none) = 0;
+  virtual MojoResult ConsumerBeginReadDataImplNoLock(
+      UserPointer<const void*> buffer,
+      UserPointer<uint32_t> buffer_num_bytes,
+      uint32_t min_num_bytes_to_read) = 0;
   virtual MojoResult ConsumerEndReadDataImplNoLock(uint32_t num_bytes_read) = 0;
   // Note: A consumer should not be writable during a two-phase read.
   virtual HandleSignalsState ConsumerGetHandleSignalsStateNoLock() const = 0;