Tune snapshot taking progress indicator.

Tune snapshot taking progress indicator.

As of dominators and retained sizes calculation take quite small time now
comparing to the main passes, it is worth to exclude these from progress
indicator. Now the indicator smoothly runs to 100%, while previously
it ran to 50% and then instantly jumped to 100%.

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9463008
Patch from Alexei Filippov <alexeif@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10823 ce2b1a6d-e550-0410-aec6-3dcde31c8c00