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/stylesheets/document_rule.rs

@@ -9,10 +9,10 @@
 use cssparser::{Parser, Token, SourceLocation, BasicParseError};
 use media_queries::Device;
 use parser::{Parse, ParserContext};
+use servo_arc::Arc;
 use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
 use std::fmt;
 use style_traits::{ToCss, ParseError, StyleParseError};
-use stylearc::Arc;
 use stylesheets::CssRules;
 use values::specified::url::SpecifiedUrl;