Return start/end profiling time in microseconds instead of milliseconds

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

Committed: https://code.google.com/p/v8/source/detail?r=16067

[yurys, 2013-08-05 12:36:56]

This is the same as r16049 but with the Win32 compilation fixed (type of
time_before_profiling in the test changed from double to int64_t).

[Benedikt Meurer, 2013-08-05 12:40:27]

Forwarding to danno

[yurys, 2013-08-06 07:04:02]

@danno: can you take a look please? I've got some Blink changes pending this CL.

[Benedikt Meurer, 2013-08-06 07:41:06]

LGTM

[yurys, 2013-08-06 08:01:05]

Committed patchset #1 manually as r16067 (presubmit successful).