style: Refactor the author sheet cache to keep alive the relevant StylesheetContents

This prevents incorrectly reusing cached results when the contents go
away and new contents are allocated with the same address.

Note that these keep alive transitively everything else under them, so
all other medialist keys don't need this.

By making this a proper hashmap it should also improve cache lookup
times if the cache grows too big.

Differential Revision: https://phabricator.services.mozilla.com/D115202

components/servo_arc/lib.rs

@@ -248,6 +248,14 @@ impl<T> Arc<T> {
         }
     }
 
+    /// Like from_raw, but returns an addrefed arc instead.
+    #[inline]
+    pub unsafe fn from_raw_addrefed(ptr: *const T) -> Self {
+        let arc = Self::from_raw(ptr);
+        mem::forget(arc.clone());
+        arc
+    }
+
     /// Create a new static Arc<T> (one that won't reference count the object)
     /// and place it in the allocation provided by the specified `alloc`
     /// function.