Auto merge of #15906 - mchv:mchv-safe-browsing-context, r=asajeffrey

Access browsing context safely

Current browsing context accessor in `Document` unwraps the browsing context `Option` which prevents document to handle correctly the case when there is no browsing context.

This is the reason servo panics with `session-history.max-length=1` (https://github.com/servo/servo/issues/15877).

As it is my first contribution, I added a `safe` method to retrieve the browsing context rather than change the existing method, but I am happy to change if you think this is the right approach. I did not as well replace all existing method call to the `safe` method, to focus on fixing the issue.

If someone can give me a bit of guidance for the test, I will try to contribute one.

---
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- [X] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [X] These changes fix #15877.
- [ ] There are tests for these changes

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components/script/dom/document.rs

@@ -398,7 +398,7 @@ impl Document {
     #[inline]
     pub fn browsing_context(&self) -> Option<Root<BrowsingContext>> {
         if self.has_browsing_context {
-            Some(self.window.browsing_context())
+            self.window.maybe_browsing_context()
         } else {
             None
         }

components/script/dom/window.rs

@@ -319,10 +319,15 @@ impl Window {
         &self.image_cache_thread
     }
 
+    /// This can panic if it is called after the browsing context has been discarded
     pub fn browsing_context(&self) -> Root<BrowsingContext> {
         self.browsing_context.get().unwrap()
     }
 
+    pub fn maybe_browsing_context(&self) -> Option<Root<BrowsingContext>> {
+        self.browsing_context.get()
+    }
+
     pub fn bluetooth_thread(&self) -> IpcSender<BluetoothRequest> {
         self.bluetooth_thread.clone()
     }
@@ -627,7 +632,7 @@ impl WindowMethods for Window {
     // https://html.spec.whatwg.org/multipage/#dom-parent
     fn GetParent(&self) -> Option<Root<BrowsingContext>> {
         // Steps 1. and 2.
-        if self.browsing_context().is_discarded() {
+        if self.maybe_browsing_context().map_or(true, |c| c.is_discarded()) {
             return None;
         }
         match self.parent() {
@@ -641,7 +646,7 @@ impl WindowMethods for Window {
     // https://html.spec.whatwg.org/multipage/#dom-top
     fn GetTop(&self) -> Option<Root<BrowsingContext>> {
         // Steps 1. and 2.
-        if self.browsing_context().is_discarded() {
+        if self.maybe_browsing_context().map_or(true, |c| c.is_discarded()) {
             return None;
         }
         // Step 5.