This is preparation to allow for a non-sampling events processor which
receives ticks from a source not driven by a timer. This will allow us
to have more deterministic testing of the CPU profiler.

It also allows different implementations for a wall time and CPU time
triggered sampler.

Change-Id: I2e9db9580ec70f05094e59c2c1e5efc28c8f7da8
Reviewed-on: https://chromium-review.googlesource.com/c/1280436Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56717}