Return start/end profiling time in microseconds instead of milliseconds
The start and end time are now measured in microseconds and the type is int64_t.
This way it seems more natural as we are going to support submilisecond sampling
rate soon. Also it fixes cctest/test-cpu-profiler/ProfileStartEndTime test
failure caused by comparison between long double and double.
TEST=cctest/test-cpu-profiler/ProfileStartEndTime
BUG=v8:2824
R=bmeurer@chromium.org
Review URL: https://codereview.chromium.org/
22155003
git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16067
ce2b1a6d-e550-0410-aec6-
3dcde31c8c00