Remove -webkit-mask-*.

Remove -webkit-mask-*.

This is for doing masking based off the alpha channel of
an image. This is a feature we want to support, but we
want a more general imperative API that the declarative thing
is built on top of.

In the meantime, the code is getting in our way and the
feature was already broken (likely from before making sky public).
We just paint the mask image on top instead of doing the actual
masking.

This patch just removes the parsing. Followup patches will
remove the implementation code.

R=esprehn@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/9c79fb7d4e0fdddb71dedcc7c15e3b4b099c0954

[esprehn, 2015-02-02 02:41:04]

esprehn@chromium.org changed reviewers:
+ esprehn@chromium.org

[esprehn, 2015-02-02 02:41:05]

lgtm

[ojan, 2015-02-02 02:45:49]

Committed patchset #2 (id:20001) manually as
9c79fb7d4e0fdddb71dedcc7c15e3b4b099c0954 (presubmit successful).