Auto merge of #11791 - craftytrickster:11712/pipeline-lookup, r=asajeffrey

Pipeline lookup in webdriver

Fixes #11712

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Removed a method that seemed to duplicate already existing functionality, and returned BrowsingContextErrors in the web_handler file where panics were previously occurring.

I am not sure if I like all the unwrapping that occurs in the script thread, but the current methods are not set up to return Option/Result.

Also, should line the method on line 37 `find_node_by_unique_id` of components/script/webdriver_handlers.rs return None if the context is not found like I have it currently? Or should it return a `Result<Option...>` instead?

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11712 .

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because I simply removed a method that duplicated already existing functionality. On the other part, I added better error sending instead of forcing a panic, which does not require testing to my knowledge.

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components/script/script_thread.rs

@@ -1073,7 +1073,10 @@ impl ScriptThread {
 
     fn handle_resize(&self, id: PipelineId, size: WindowSizeData, size_type: WindowSizeType) {
         if let Some(ref context) = self.find_child_context(id) {
-            let window = context.active_window();
+            let window = match context.find(id) {
+                Some(browsing_context) => browsing_context.active_window(),
+                None => return warn!("Message sent to closed pipeline {}.", id),
+            };
             window.set_resize_event(size, size_type);
             return;
         }
@@ -2211,15 +2214,6 @@ fn shut_down_layout(context_tree: &BrowsingContext) {
     }
 }
 
-// TODO: remove this function, as it's a source of panic.
-pub fn get_browsing_context(context: &BrowsingContext,
-                            pipeline_id: PipelineId)
-                            -> Root<BrowsingContext> {
-    context.find(pipeline_id).expect("ScriptThread: received an event \
-            message for a layout channel that is not associated with this script thread.\
-            This is a bug.")
-}
-
 fn dom_last_modified(tm: &Tm) -> String {
     tm.to_local().strftime("%m/%d/%Y %H:%M:%S").unwrap().to_string()
 }