Bug 1937785 - Remove nsPresContext::mKeyboardHeight and introduce MobileViewportManager::mPendingKeyboardHeight. r=botond

There's a race condition where the keyboard height change notification
has arrived but nsDocumentViewer size change hasn't yet arrived.

MobileViewportManager::mPendingKeyboardHeight is introduced to fix the race.

Differential Revision: https://phabricator.services.mozilla.com/D234262

layout/base/PresShell.cpp

@@ -11418,7 +11418,11 @@ void PresShell::MaybeRecreateMobileViewportManager(bool aAfterInitialization) {
           ("Created MVM %p (type %d) for URI %s", mMobileViewportManager.get(),
            (int)*mvmType, uri ? uri->GetSpecOrDefault().get() : "(null)"));
     }
+    if (BrowserChild* browserChild = BrowserChild::GetFrom(this)) {
+      mMobileViewportManager->UpdateKeyboardHeight(browserChild->GetKeyboardHeight());
+    }
   }
+
   if (aAfterInitialization) {
     // Setting the initial viewport will trigger a reflow.
     if (mMobileViewportManager) {