Speedup dominators construction in heap snapshot.

Speedup dominators construction in heap snapshot.

It is achieved by:
1. skipping entries those dominators have already reached root.
2. processing only entries those retainers have changed their
   dominators and skipping other entries.
3. removing extra memory indirection by making the dominators array
   contain entry indices instead of entries themselves.

The dominators building time has dropped from ~4000 ms to ~200 ms
on gmail.com heap snapshot.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9372105
Patch from Alexei Filippov <alexeif@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10799 ce2b1a6d-e550-0410-aec6-3dcde31c8c00