Heap explorer: Show representation of strings.

Heap explorer: Show representation of strings.

* src/profile-generator.cc (V8HeapExplorer::GetSystemEntryName): For
  maps of strings, return a name that encodes the representation
  strategy used for the strings.
  (V8HeapExplorer::ExtractReferences): For strings, visit the maps as
  well, so that the user can see what representation is used for their
  strings.

See http://wingolog.org/pub/string-representations.png for an example of what it looks like.

BUG=

Review URL: https://chromiumcodereview.appspot.com/11344021
Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12973 ce2b1a6d-e550-0410-aec6-3dcde31c8c00