v2 UI should have buttons to breakdown a test

v2 UI should have buttons to breakdown a test
https://bugs.webkit.org/show_bug.cgi?id=146010

Reviewed by Chris Dumez.

Added buttons beneath each chart pane to add "alternative panes". By default, it shows every platform
as well as "Breakdown" to add all subtests' metrics.

Also removed the metric submenu from tests that had exactly one metric. When a test only measures Time
for example, we make the test itself clickable instead of showing a submenu that only contains one item.

* public/v2/app.js:
(App.ChartsController.addAlternativePanes): Added.
(App.TestProxyForPopup.children): Calls _updateChildren and returns this._children.
(App.TestProxyForPopup.actionName): Added.
(App.TestProxyForPopup.actionArgument): Added.
(App.TestProxyForPopup._updateChildren): Extracted from children. Now also sets _actionName and
_actionArgument in the case there was exactly one metric so that showing submenu is unnecessary.
(App.PaneController.alternativePanes): Added. Returns the list of alternative panes. The platform list
excludes ones that don't have this metric (e.g. iOS doesn't have desktop PLT results) as well as ones
that are already present in the list of panes.
* public/v2/chart-pane.css: Added CSS rules for alternative pane buttons beneath the chart panes.
* public/v2/index.html:
* public/v2/manifest.js:
(App.Metric.childMetrics): Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@185594 268f45cc-cd09-0410-ab3c-d52691b4dbfc