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/signal_catcher.cc b/src/signal_catcher.cc
index be21372..dd31fa7 100644
--- a/src/signal_catcher.cc
+++ b/src/signal_catcher.cc
@@ -114,7 +114,7 @@
     os << "Cmd line: " << cmdline << "\n";
   }
 
-  runtime->Dump(os);
+  runtime->DumpForSigQuit(os);
 
   if (false) {
     std::string maps;