Replace all uses of the style::stylearc alias with servo_arc.

The alias is left there temporarilly and will be removed completely in a later commit where
also components/style/gecko/generated/structs_{debug|release}.rs are re-generated (they still
use the old alias).

components/style/shared_lock.rs

@@ -8,11 +8,11 @@
 use atomic_refcell::{AtomicRefCell, AtomicRef, AtomicRefMut};
 #[cfg(feature = "servo")]
 use parking_lot::RwLock;
+use servo_arc::Arc;
 use std::cell::UnsafeCell;
 use std::fmt;
 #[cfg(feature = "gecko")]
 use std::ptr;
-use stylearc::Arc;
 
 /// A shared read/write lock that can protect multiple objects.
 ///