Fixes <http://webkit.org/b/31700>.

Fixes <http://webkit.org/b/31700>.
Web Inspector: isMac should be in one central location + Cached.

Reviewed by Tim Hatcher.

We should cache the value of isMac, and make the indexOf call in one
single location, because we are calling it from multiple places, and saving
the result can save us time, and make the call sites simpler.

* inspector/front-end/AbstractTimelinePanel.js:
(WebInspector.AbstractTimelinePanel.prototype._updateFilter):
* inspector/front-end/ConsoleView.js:
(WebInspector.ConsoleView.prototype._updateFilter):
* inspector/front-end/ElementsPanel.js:
(WebInspector.ElementsPanel.prototype.handleKeyEvent):
* inspector/front-end/ScriptsPanel.js:
(WebInspector.ScriptsPanel):
* inspector/front-end/SourceFrame.js:
(WebInspector.SourceFrame.prototype._loaded):
* inspector/front-end/inspector.js:
(WebInspector.documentKeyDown):
(WebInspector.isMac):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@51250 268f45cc-cd09-0410-ab3c-d52691b4dbfc