Sync CSSStyleDeclaration.setProperty with the spec [1], specifically
make value and property not nullable, treat null as empty string
and for value change default from null to empty string.

TEST: LayoutTests/fast/css/cssom-setproperty-undefined-value.html

[1] https://drafts.csswg.org/cssom/#the-cssstyledeclaration-interface

Change-Id: I9ca067626c65aed5cce7f9bcb23af1133a0822b6
Reviewed-on: https://chromium-review.googlesource.com/806751
Commit-Queue: Rob Buis <[email protected]>
Reviewed-by: Philip Jägenstedt <[email protected]>
Cr-Commit-Position: refs/heads/master@{#523726}