Revert of DevTools: notify backend agents about profiler started/stopped events (https://codereview.chromium.org/321173002/)

Reason for revert:
We came to the conclusion that this patch
moves us in a wrong direction. 
With this approach we would have to manage 4 pairs of states. Disabled+stopped, disabled+started, enabled+stopped, etc up to enabled+started :)
in the each agent. So we would like to try the simplest one, just use enable/disable states and fix the problems related to them.

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175946

TBR=pfeldman@chromium.org,vsevik@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=381115

Review URL: https://codereview.chromium.org/327203002

git-svn-id: svn://svn.chromium.org/blink/trunk@175954 bbb929c8-8fbe-4397-9dbb-9b2b20218538