style: Make webkit device-pixel-ratio media queries a proper alias to resolution.

According to the spec:

  https://compat.spec.whatwg.org/#css-media-queries-webkit-device-pixel-ratio

And to the Chromium implementation:

  https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/css/media_query_evaluator.cc?l=366&rcl=1d7328865bcf06a687aafc18ff95d55317030672

They're no different than resolution.

In our implementation `resolution` does slightly different stuff. Given we
still haven't shipped -webkit-device-pixel-ratio, making this match resolution
looks better than the opposite.

Differential Revision: https://phabricator.services.mozilla.com/D3588

components/style/values/computed/resolution.rs

@@ -21,6 +21,12 @@ impl Resolution {
     pub fn dppx(&self) -> CSSFloat {
         self.0
     }
+
+    /// Return a computed `resolution` value from a dppx float value.
+    #[inline]
+    pub fn from_dppx(dppx: CSSFloat) -> Self {
+        Resolution(dppx)
+    }
 }
 
 impl ToComputedValue for specified::Resolution {