Add GetPropertyPriority to CSSStyleDeclaration

Tweak getPropertyPriority to match recommendations

Adding Tests for Style Priority

Use else if

Adding Content Test for GetPropertyPriority

Revert "Adding Tests for Style Priority"

This reverts commit 8666a37f833b06c3e43f27acd8a9678e94425e55.
auto merge of #4463 : iterion/servo/get-property-priority, r=jdm

Implementation of #4432 adding `getPropertyPriority` to CSSStyleDeclaration. This is my first attempt at a Servo PR so I'm sure I've done something wrong. Let me know, and I'll fix it up.

As stated in #4432 tests for this are in #4085. If there are additional tests I can write now I would love to do that, I'm just not sure where or what those would be.