Note that this changes the sampling interval from milliseconds to
microseconds -- this shouldn't cause issues except for tools that use
'profiler,"begin",<interval>' somehow.

Change-Id: I20222de91f7820e26eb3fc505a4752b0bc7e1642
Reviewed-on: https://chromium-review.googlesource.com/451658
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43726}