Bug 1574307 - Part 3. Dispatch DOMMetaViewportFitChanged when viewport-fit is changed. r=smaug,hiro

Actually, we have `DOMMeta*` event for modifying meta element. But GeckoView wants viewport-fit change only.

@hiro suggests new event that is fired by viewport-fit will be changed. This event doesn't need current viewport-fit value since GeckoView doesn't need current value immediately.

Depends on D57397

Differential Revision: https://phabricator.services.mozilla.com/D57398

--HG--
extra : moz-landing-system : lando

dom/base/Document.cpp

@@ -9875,6 +9875,11 @@ void Document::AddMetaViewportElement(HTMLMetaElement* aElement,
   mMetaViewports.AppendElement(MetaViewportElementAndData{aElement, aData});
   // Trigger recomputation of the nsViewportInfo the next time it's queried.
   mViewportType = Unknown;
+
+  RefPtr<AsyncEventDispatcher> asyncDispatcher = new AsyncEventDispatcher(
+      this, NS_LITERAL_STRING("DOMMetaViewportFitChanged"), CanBubble::eYes,
+      ChromeOnlyDispatch::eYes);
+  asyncDispatcher->RunDOMEventWhenSafe();
 }
 
 void Document::RemoveMetaViewportElement(HTMLMetaElement* aElement) {
@@ -9883,6 +9888,11 @@ void Document::RemoveMetaViewportElement(HTMLMetaElement* aElement) {
       mMetaViewports.RemoveElement(viewport);
       // Trigger recomputation of the nsViewportInfo the next time it's queried.
       mViewportType = Unknown;
+
+      RefPtr<AsyncEventDispatcher> asyncDispatcher = new AsyncEventDispatcher(
+          this, NS_LITERAL_STRING("DOMMetaViewportFitChanged"), CanBubble::eYes,
+          ChromeOnlyDispatch::eYes);
+      asyncDispatcher->RunDOMEventWhenSafe();
       return;
     }
   }