This CL adds a very crude unittest to check that RuntimeCallStats work
correctly with api callbacks present. This currently doesn't check that
all parent timers (namely FunctionCallback) are handled properly.

Drive-by-Fix:
- Use Microseconds for all RCS timer tests
- Add TestWithContext::SetGlobalProperty helper
- Use explicit v8:: prefix in test-utils.{h,cc}

Change-Id: I054e78abca0b87a3b9e07d3b06cccdad15403bae
Reviewed-on: https://chromium-review.googlesource.com/766429
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49348}