Mojo: Implement MojoUnmapBuffer().

mojo/system/constants.h

Index: mojo/system/constants.h
diff --git a/mojo/system/constants.h b/mojo/system/constants.h
index 6c287b530b773338739251d00aa8afc934094869..b92eef102218e4463d15f78bc28f9b4863706c90 100644
--- a/mojo/system/constants.h
+++ b/mojo/system/constants.h
@@ -10,11 +10,14 @@
 namespace mojo {
 namespace system {
 
-// Maximum of open (Mojo) handles.
+// Maximum number of open (Mojo) handles.
 // TODO(vtl): This doesn't count "live" handles, some of which may live in
 // messages.
 const size_t kMaxHandleTableSize = 1000000;
 
+// Maximum number of active memory mappings.
+const size_t kMaxMappingTableSize = 1000000;
+
 const size_t kMaxWaitManyNumHandles = kMaxHandleTableSize;
 
 const size_t kMaxMessageNumBytes = 4 * 1024 * 1024;