[RootLayerScrolls] Remove internal LFV check in ScrollableAreaForNode

This internals-only code is not needed post-rls.

BUG=823365

Change-Id: I48e9a149230a89bc235e3323a132420764fbc394
Reviewed-on: https://chromium-review.googlesource.com/1154321Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580333}

third_party/blink/renderer/core/testing/internals.cc

@@ -242,12 +242,6 @@ static ScrollableArea* ScrollableAreaForNode(Node* node) {
   if (!node)
     return nullptr;
 
-  if (node->IsDocumentNode()) {
-    // This can be removed after root layer scrolling is enabled.
-    if (LocalFrameView* frame_view = ToDocument(node)->View())
-      return frame_view->LayoutViewport();
-  }
-
   LayoutObject* layout_object = node->GetLayoutObject();
   if (!layout_object || !layout_object->IsBox())
     return nullptr;