Calling offsetWidth in a style-modifying forEach is slow

[ajklein]

g-flow-panel-transition.html queries offsetWidth in a forEach loop, then modifies style. This will cause relayouts each time through the function:

https://github.com/Polymer/toolkit-ui/blob/master/elements/panel-transitions/g-flow-panel-transition.html#L119

(also in applyRightLayout, a little further down). I'm not clear on how many panels getPanels() is likely to return, but if it's more than 1 or 2 this is likely to be noticeable (I see it in a v8 profile, but again this should be taken with a grain of salt).

[sorvell]

This has been optimized away.