Include a new heap summary line in the SIGQUIT output.

Looks like this:

  Heap: 87% free, 4MB/32MB; 6327 objects

While I'm here, fix another long-standing TODO to make PrettySize have the
usual google3 behavior. (I took the specific thresholds from Chromium.)

Also distinguish between the more general "Dump" member functions and the
specific SIGQUIT-related ones by consistently calling the latter DumpForSigQuit.

Change-Id: I76e783adc18dd089bac9b348f53dc9860a0fe4b9
diff --git a/src/thread_list.h b/src/thread_list.h
index 0691808..f0b4f6b 100644
--- a/src/thread_list.h
+++ b/src/thread_list.h
@@ -31,7 +31,7 @@
   explicit ThreadList();
   ~ThreadList();
 
-  void Dump(std::ostream& os);
+  void DumpForSigQuit(std::ostream& os);
   void DumpLocked(std::ostream& os); // For thread suspend timeout dumps.
   pid_t GetLockOwner(); // For SignalCatcher.