style: Don't look for viewport units in stylesheets.

Use whether we've computed any viewport unit instead.

This is more accurate (we avoid restyling unnecessarily if we've found anything
ever on the stylesheet, but that hasn't matched).

This has the benefit of also matching Gecko, and simplify some code and
fishyness around, and also hopefully speeding up stylesheet parsing.

components/style/servo/media_queries.rs

@@ -48,21 +48,26 @@ pub struct Device {
     /// by using rem units.
     #[ignore_heap_size_of = "Pure stack type"]
     used_root_font_size: AtomicBool,
+    /// Whether any styles computed in the document relied on the viewport size.
+    #[ignore_heap_size_of = "Pure stack type"]
+    used_viewport_units: AtomicBool,
 }
 
 impl Device {
     /// Trivially construct a new `Device`.
-    pub fn new(media_type: MediaType,
-               viewport_size: TypedSize2D<f32, CSSPixel>,
-               device_pixel_ratio: ScaleFactor<f32, CSSPixel, DevicePixel>)
-               -> Device {
+    pub fn new(
+        media_type: MediaType,
+        viewport_size: TypedSize2D<f32, CSSPixel>,
+        device_pixel_ratio: ScaleFactor<f32, CSSPixel, DevicePixel>
+    ) -> Device {
         Device {
-            media_type: media_type,
-            viewport_size: viewport_size,
-            device_pixel_ratio: device_pixel_ratio,
+            media_type,
+            viewport_size,
+            device_pixel_ratio,
             // FIXME(bz): Seems dubious?
             root_font_size: AtomicIsize::new(font_size::get_initial_value().value() as isize),
             used_root_font_size: AtomicBool::new(false),
+            used_viewport_units: AtomicBool::new(false),
         }
     }
 
@@ -98,10 +103,15 @@ impl Device {
                     Au::from_f32_px(self.viewport_size.height))
     }
 
-    /// Returns the viewport size in pixels.
-    #[inline]
-    pub fn px_viewport_size(&self) -> TypedSize2D<f32, CSSPixel> {
-        self.viewport_size
+    /// Like the above, but records that we've used viewport units.
+    pub fn au_viewport_size_for_viewport_unit_resolution(&self) -> Size2D<Au> {
+        self.used_viewport_units.store(true, Ordering::Relaxed);
+        self.au_viewport_size()
+    }
+
+    /// Whether viewport units were used since the last device change.
+    pub fn used_viewport_units(&self) -> bool {
+        self.used_viewport_units.load(Ordering::Relaxed)
     }
 
     /// Returns the device pixel ratio.