When counting GCs in idle round, count only GC that were directly triggered by the idle notification handler.

R=hpayer@chromium.org
BUG=241815

Review URL: https://chromiumcodereview.appspot.com/15737007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14879 ce2b1a6d-e550-0410-aec6-3dcde31c8c00