Ensure layout/script always have a correct viewport size when a new pipeline is created.

components/constellation/session_history.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use crate::browsingcontext::NewBrowsingContextInfo;
+use euclid::Size2D;
 use msg::constellation_msg::{
     BrowsingContextId, HistoryStateId, PipelineId, TopLevelBrowsingContextId,
 };
@@ -10,6 +11,7 @@ use script_traits::LoadData;
 use servo_url::ServoUrl;
 use std::cmp::PartialEq;
 use std::{fmt, mem};
+use style_traits::CSSPixel;
 
 /// Represents the joint session history
 /// https://html.spec.whatwg.org/multipage/#joint-session-history
@@ -122,6 +124,9 @@ pub struct SessionHistoryChange {
     /// Holds data for not-yet constructed browsing contexts that are not
     /// easily available when they need to be constructed.
     pub new_browsing_context_info: Option<NewBrowsingContextInfo>,
+
+    /// The size of the viewport for the browsing context.
+    pub window_size: Size2D<f32, CSSPixel>,
 }
 
 /// Represents a pipeline or discarded pipeline in a history entry.