WebKitPerfMonitor: Tooltips cannot be pinned after using browser's back button

WebKitPerfMonitor: Tooltips cannot be pinned after using browser's back button
https://bugs.webkit.org/show_bug.cgi?id=131155

Reviewed by Andreas Kling.

The bug was caused by Chart.attach binding event listeners on plot container on each call.
This resulted in the click event handler toggling the visiblity of the tooltip twice upon
click when attach() has been called even number of times, keeping the tooltip invisible.

Fixed the bug by extracting the code to bind event listeners outside of Chart.attach as
a separate function, bindPlotEventHandlers, and calling it exactly once when Chart.attach
is called for the first time.

* public/index.html:
(Chart.attach):
(Chart..bindPlotEventHandlers):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@166723 268f45cc-cd09-0410-ab3c-d52691b4dbfc