Add a percentage to each metric in the V8 timeline plot

With the percentage, we can easily figure out how much execution time is spent in what metric.

c.f. https://docs.google.com/a/google.com/file/d/0B7GPzKNdEGeQU04ySy1qOWNuLWs/edit

BUG=

Review URL: https://chromiumcodereview.appspot.com/11975045
Patch from Kentaro Hara <haraken@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13415 ce2b1a6d-e550-0410-aec6-3dcde31c8c00