checkbox mask create issue under flex box

Please include the following information with your bug report:

> What MDL Version are you using? (please be specific, e.g. _major.minor.patch_)
1.3

> What browser(s) is this bug affecting (including version)?
Chrome/FF/Safari

> What OS (and version) are you using?
MACOS

> What are the steps to reproduce the bug? Can you create a plunker/codepen/jsfiddle which reproduces it?
https://codepen.io/surinder/pen/KvjJVe

> What is the expected behavior?

> What is the actual behavior?

> Any other information you believe would be useful?
fixed using `.mdl-checkbox__tick-outline{-webkit-mask:none;mask:none;}` :)