Switch CPU profile start/stop markers to monotonic time.

LOG=N
BUG=363976
R=bmeurer@chromium.org, yurys@chromium.org

Review URL: https://codereview.chromium.org/243033002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20866 ce2b1a6d-e550-0410-aec6-3dcde31c8c00