Refactor Profiler::Frame so subclasses of Profiler can have their own Fr...

Summary: ...ame.

Change allocation/deallocation of Profiler::Frame to use new/delete.
Add a virtual destructor to Profiler::Frame.
Fiddle with ASCII art.
Closes https://github.com/facebook/hhvm/pull/4305

Reviewed By: @bertmaher

Differential Revision: D1697375

Signature: t1:1697375:1417580772:001c934cd4a863cdcbe418a580faf3521222b801