Bug 1534255 - Enable out-of-process iframes to take APZ focus. r=rhunt

Differential Revision: https://phabricator.services.mozilla.com/D22968

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

gfx/layers/apz/src/FocusTarget.cpp

@@ -6,6 +6,7 @@
 
 #include "mozilla/layers/FocusTarget.h"
 
+#include "mozilla/dom/BrowserBridgeChild.h"  // for BrowserBridgeChild
 #include "mozilla/dom/EventTarget.h"     // for EventTarget
 #include "mozilla/dom/TabParent.h"       // for TabParent
 #include "mozilla/EventDispatcher.h"     // for EventDispatcher
@@ -179,6 +180,17 @@ FocusTarget::FocusTarget(nsIPresShell* aRootPresShell,
     return;
   }
 
+  // Check if the key event target is a remote browser
+  if (BrowserBridgeChild* bbc = BrowserBridgeChild::GetFrom(keyEventTarget)) {
+    FT_LOG("Creating oopif reflayer target with seq=%" PRIu64
+           ", kl=%d, lt=%" PRIu64 "\n",
+           aFocusSequenceNumber, mFocusHasKeyEventListeners,
+           bbc->GetLayersId());
+
+    mData = AsVariant<LayersId>(bbc->GetLayersId());
+    return;
+  }
+
   // The content to scroll is either the focused element or the focus node of
   // the selection. It's difficult to determine if an element is an interactive
   // element requiring async keyboard scrolling to be disabled. So we only