Provide method to dump backtrace heap data.

For non-zygote spawned processes, we might want to dump the backtrace
data. Provide a method to send a signal to a process and then dump the
data to a file.

Adds a method to dump the backtrace data on exit.

Update documentation and explain format of heap dump data.

Test: Ran unit tests, enabled new options and used them.
Change-Id: Ie2fa706694160731afe02c1382b037d06df1d069
diff --git a/libc/malloc_debug/TrackData.h b/libc/malloc_debug/TrackData.h
index e4c8951..f7486e9 100644
--- a/libc/malloc_debug/TrackData.h
+++ b/libc/malloc_debug/TrackData.h
@@ -51,6 +51,8 @@
 
   void GetList(std::vector<const Header*>* list);
 
+  void GetListBySizeThenBacktrace(std::vector<const Header*>* list, size_t* total_memory);
+
   void Add(const Header* header, bool backtrace_found);
 
   void Remove(const Header* header, bool backtrace_found);