Auto merge of #14367 - mrobinson:scroll-fragment-point, r=pcwalton

Reimplement scrolling to fragments

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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 #13736, #10753 (github issue number if applicable).

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This reimplemntation of the feature uses ScrollRootIds to scroll
particular scrollable areas of the page.

Fixes #13736.
Fixes #10753.

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components/layout/flow.rs

@@ -406,7 +406,10 @@ pub trait Flow: fmt::Debug + Sync + Send + 'static {
     /// Print any extra children (such as fragments) contained in this Flow
     /// for debugging purposes. Any items inserted into the tree will become
     /// children of this flow.
-    fn print_extra_flow_children(&self, _: &mut PrintTree) {
+    fn print_extra_flow_children(&self, _: &mut PrintTree) { }
+
+    fn scroll_root_id(&self) -> ScrollRootId {
+        base(self).scroll_root_id
     }
 }