Add support for parsing and applying viewport <meta> (#35901)

This patch comprises following steps:
1. Parses the `viewport` Attribute from `<meta>`.
2. Creates a `ViewportDescription` struct.
3. Populate values into Viewport struct.
4. Pass & Stash Viewport Description to Webview.
5. Process parsed values of `viewport <meta>`

Testing: Tested locally.
Fixes: #36159

---------

Signed-off-by: Shubham Gupta <shubham13297@gmail.com>
Signed-off-by: Xiaocheng Hu <xiaochengh.work@gmail.com>
Co-authored-by: Xiaocheng Hu <xiaochengh.work@gmail.com>

components/script/dom/htmlmetaelement.rs

@@ -2,6 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use std::str::FromStr;
+
+use compositing_traits::CompositorMsg;
+use compositing_traits::viewport_description::ViewportDescription;
 use dom_struct::dom_struct;
 use html5ever::{LocalName, Prefix, local_name, ns};
 use js::rust::HandleObject;
@@ -61,6 +65,9 @@ impl HTMLMetaElement {
             if name == "referrer" {
                 self.apply_referrer();
             }
+            if name == "viewport" {
+                self.parse_and_send_viewport_if_necessary();
+            }
         // https://html.spec.whatwg.org/multipage/#attr-meta-http-equiv
         } else if !self.HttpEquiv().is_empty() {
             // TODO: Implement additional http-equiv candidates
@@ -115,6 +122,29 @@ impl HTMLMetaElement {
         }
     }
 
+    /// <https://drafts.csswg.org/css-viewport/#parsing-algorithm>
+    fn parse_and_send_viewport_if_necessary(&self) {
+        // Skip processing if this isn't the top level frame
+        if !self.owner_window().is_top_level() {
+            return;
+        }
+        let element = self.upcast::<Element>();
+        let Some(content) = element.get_attribute(&ns!(), &local_name!("content")) else {
+            return;
+        };
+
+        if let Ok(viewport) = ViewportDescription::from_str(&content.value()) {
+            self.owner_window()
+                .compositor_api()
+                .sender()
+                .send(CompositorMsg::Viewport(
+                    self.owner_window().webview_id(),
+                    viewport,
+                ))
+                .unwrap();
+        }
+    }
+
     /// <https://html.spec.whatwg.org/multipage/#attr-meta-http-equiv-content-security-policy>
     fn apply_csp_list(&self) {
         if let Some(parent) = self.upcast::<Node>().GetParentElement() {