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/utils.h b/src/utils.h
index f11a769..f94c05e 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -191,7 +191,7 @@
 // Returns a human-readable form of the name of the given class with its class loader.
 std::string PrettyClassAndClassLoader(const Class* c);
 
-// Returns a human-readable size string. e.g. "1MB"
+// Returns a human-readable size string such as "1MB".
 std::string PrettySize(size_t size_in_bytes);
 
 // Returns a human-readable time string which prints every nanosecond while trying to limit the