-e: WebGL slider is black in build if handle or background has no source image

-repro:
--open scene "BlackHandle"
--press play
--notice that the slider has a red colored handle and a yellow handle slide area
--build scene as WebGL
--open WebGL scene and notice that the slider handle and slide area are black

Workaround: Add a white sprite to handle source image and background

Used to work in 5.1.4 and also works in standalone build.

Maybe related to https://fogbugz.unity3d.com/default.asp?729887