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/constellation/constellation.rs

@@ -1038,8 +1038,9 @@ impl<Message, LTF, STF> Constellation<Message, LTF, STF>
                 self.handle_alert(pipeline_id, message, sender);
             }
 
-            FromScriptMsg::ScrollFragmentPoint(pipeline_id, point, smooth) => {
+            FromScriptMsg::ScrollFragmentPoint(pipeline_id, scroll_root_id, point, smooth) => {
                 self.compositor_proxy.send(ToCompositorMsg::ScrollFragmentPoint(pipeline_id,
+                                                                                scroll_root_id,
                                                                                 point,
                                                                                 smooth));
             }