csstext
- [x] These changes fix#4431.
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy --faster` reports one error for the `css_properties_accessors!` macro not having a spec link
```
$ git log -1 --format=oneline
91fb9bf1d3 fixup! implement cssText
$ ./mach build -d
Compiling style v0.0.1 (file:///Users/greg/servo/components/style)
Compiling gfx v0.0.1 (file:///Users/greg/servo/components/gfx)
Compiling script v0.0.1 (file:///Users/greg/servo/components/script)
Compiling layout_traits v0.0.1 (file:///Users/greg/servo/components/layout_traits)
Compiling compositing v0.0.1 (file:///Users/greg/servo/components/compositing)
Compiling glutin_app v0.0.1 (file:///Users/greg/servo/ports/glutin)
Compiling constellation v0.0.1 (file:///Users/greg/servo/components/constellation)
Compiling layout v0.0.1 (file:///Users/greg/servo/components/layout)
Compiling servo v0.0.1 (file:///Users/greg/servo/components/servo)
Build completed in 0:05:11.475584
$ ./mach test-tidy --faster
Checking files for tidiness...
./components/script/dom/cssstyledeclaration.rs:386: method declared in webidl is missing a comment with a specification link
Progress: 100% (12/12)
```
- [x] There are tests for these changes. More CSSOM tests pass, but others fail that probably shouldn't:
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/index-002.htm` and a bunch of the other tests in `/css-tests/cssom-1_dev/html/ crash when run individually
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/cssom-cssText-serialize.htm` fails to strip a trailing semicolon (`left: 10px` vs `left: 10px;`)
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/index-001.htm` shared shorthand values aren't coalesced (`margin: 20px` vs. `margin: 20px 20px 20px 20px`)
* `./mach test-css tests/wpt/css-tests/cssom-1_dev/html/cssstyledeclaration-csstext.htm` also crashes and fails for `color: red;` vs. `color: RED;`, preserving declaration insertion order, whitespace in the value, and setting an unknown style property.
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