Bug 1518121 - Part 1: Check isFocusable from frame first if content has frame; r=smaug

This is for the scrollable frame, because nsIFrame::IsFocusable of a scrollable
frame returns true with tabIndex = 0 even if the content has no tabIndex
attribute.

Differential Revision: https://phabricator.services.mozilla.com/D19646

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

dom/base/nsFocusManager.cpp

@@ -3146,6 +3146,8 @@ nsIContent* nsFocusManager::GetNextTabbableContentInAncestorScopes(
     int32_t tabIndex = 0;
     if (IsHostOrSlot(startContent)) {
       tabIndex = HostOrSlotTabIndexValue(startContent);
+    } else if (nsIFrame* frame = startContent->GetPrimaryFrame()) {
+      frame->IsFocusable(&tabIndex);
     } else {
       startContent->IsFocusable(&tabIndex);
     }