Add start and end profiling time to v8::CpuProfile

Add start and end profiling time to v8::CpuProfile

I'm going to change CPU profiler API and deprecate GetSelfTime, GetTotalTime and GetTotalSamplesCount on CpuProfileNode as all of those values are derived from self samples count and sampling rate. The sampling rate in turn is calculate based on the profiling duration so having start/end time and total sample count is enough for calculating smpling rate.

BUG=267595
R=alph@chromium.org, bmeurer@chromium.org

Review URL: https://codereview.chromium.org/21918002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16039 ce2b1a6d-e550-0410-aec6-3dcde31c8c00