Delete all CPU profiles and heap snapshots on front-end disconnection

* Profiler data is not preserved between front-end sessions anymore.
* {Heap}Profiler.getProfileHeaders commands were removed from the protocol. HeapProfiler.enable/disable were added to control events being sent to front-end.

BUG=306375
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/26807005

git-svn-id: svn://svn.chromium.org/blink/trunk@159586 bbb929c8-8fbe-4397-9dbb-9b2b20218538