DevTools: notify backend agents about profiler started/stopped events

This patch provides a way to notify backend agents about starting a profiler.
Each agent should switch to a mode which doesn't affect the performance.
As example DebuggerAgent needs to disable breakpoints, allow v8 optimizations, etc.
DOM and Network agents may simple disable themselves.

BUG=381115
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/321173002

git-svn-id: svn://svn.chromium.org/blink/trunk@175946 bbb929c8-8fbe-4397-9dbb-9b2b20218538