Simplify the internal state of Sampler a bit. There are basically two
users of Sampler - the CpuSampler used by the CpuProfiler and the
Ticker used by log.cc. Ticker calls Start/Stop to manage the Sampler
lifetime, but CpuProfiler does not. This leads to much confusion and
overlap of functionality.

Fix that here by removing the distinction between active, registered
and isProfiling states. These are now all the same thing and are
represented by IsActive(). The state is set to active when Start is
called, and set inactive when Stop is called. Both users of Sampler
now call Start and Stop at appropriate times.

The concept of profiling depth was not used - each Sampler would
only ever have a sampling depth of 1. We still need to call
SignalHandler::IncreaseSamplerCount(), so we do that in Start
and the corresponding DecreaseSamplerCount() in Stop.

Change-Id: I16a9435d26169a7dd00b1c7876e66af45f12e4b0
Reviewed-on: https://chromium-review.googlesource.com/c/1424337
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58955}