style: Don't EnsureUniqueInner from the cssRules getter

Instead, fix up the various content data structures when the stylesheet
is mutated. This makes reading a stylesheet not disable style sharing.

Differential Revision: https://phabricator.services.mozilla.com/D115203

components/style/gecko/data.rs

@@ -56,10 +56,17 @@ impl GeckoStyleSheet {
     /// already holds a strong reference.
     #[inline]
     pub unsafe fn from_addrefed(s: *const DomStyleSheet) -> Self {
-        debug_assert!(!s.is_null());
+        assert!(!s.is_null());
         GeckoStyleSheet(s)
     }
 
+    /// HACK(emilio): This is so that we can avoid crashing release due to
+    /// bug 1719963 and can hopefully get a useful report from fuzzers.
+    #[inline]
+    pub fn hack_is_null(&self) -> bool {
+        self.0.is_null()
+    }
+
     /// Get the raw `StyleSheet` that we're wrapping.
     pub fn raw(&self) -> &DomStyleSheet {
         unsafe { &*self.0 }

components/style/stylesheets/import_rule.rs

@@ -56,7 +56,13 @@ impl ImportSheet {
     /// exists.
     pub fn as_sheet(&self) -> Option<&crate::gecko::data::GeckoStyleSheet> {
         match *self {
-            ImportSheet::Sheet(ref s) => Some(s),
+            ImportSheet::Sheet(ref s) => {
+                debug_assert!(!s.hack_is_null());
+                if s.hack_is_null() {
+                    return None;
+                }
+                Some(s)
+            },
             ImportSheet::Pending(_) => None,
         }
     }