tcmalloc: export SetPrevious and SetNext APIs for singly linked free lists

These APIs are used by the central cache. Their absence in the exported API
of the singly linked list implementation was not noticed because doubly
linked lists are enabled for all Chromium builds.

BUG=724399,b:70905156

Change-Id: I0187789eb416ce751aa1a369ea96c0d317d3702a
Reviewed-on: https://chromium-review.googlesource.com/1195093Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Gabriel Marin <gmx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588200}

third_party/tcmalloc/chromium/src/free_list.h

@@ -179,6 +179,14 @@ inline void* FL_Pop(void** list) {
   return SLL_Pop(list);
 }
 
+inline void FL_SetPrevious(void* t, void* n) {
+  // Singly linked list has no previous element.
+}
+
+inline void FL_SetNext(void* t, void* n) {
+  SLL_SetNext(t, n);
+}
+
 // Removes |N| elements from a linked list to which |head| points.
 // |head| will be modified to point to the new |head|.  |start| and
 // |end| will point to the first and last nodes of the range.  Note