Modify SkEventTracer::SetInstance to fail rather than assert if a tracer has already...

Modify SkEventTracer::SetInstance to fail rather than assert if a tracer has already been installed.

Chromium usually calls SetInstance once per process. However, when run in single process more renderer threads will try to set the instance after the browser process already has done so. This allows them to fail gracefully without asserting.

Bug: skia:6603
Change-Id: Ic8a35422d787335aa67eefc07d0658f0fbe73db4
Reviewed-on: https://skia-review.googlesource.com/16664
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>