Currently we enable instrumentation if debugger is active. With this
approach we can not:
- capture async stack when debugger is disabled,
- avoid async instrumentation overhead when debugger is enabled and
  async stacks are disabled.

R=dgozman@chromium.org,yangguo@chromium.org

Bug: none
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;luci.chromium.try:linux_chromium_rel_ng;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I19400c4c4e12b6c9b5a980fb6bd3293bac6e6a64
Reviewed-on: https://chromium-review.googlesource.com/1081494
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53530}