Determine if RWHVMac cast is safe without checking for a BrowserPluginGuest

The GetRenderWidgetHostViewMac(NSObject*) test function was using
whether a WebContents was a guest to determine whether a cast to
RenderWidgetHostViewMac was safe. This would not be correct with other
nested WebContents, so we update the logic to check for a root RWHV
before casting.

Bug: 533069
Change-Id: I7af9fae9a862e2731e5770a1f628f9d6c39f8dc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436950Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811415}

content/public/test/content_browser_test_utils_mac.mm

@@ -55,9 +55,10 @@ namespace {
 
 content::RenderWidgetHostViewMac* GetRenderWidgetHostViewMac(NSObject* object) {
   for (auto* contents : WebContentsImpl::GetAllWebContents()) {
-    if (!contents->GetBrowserPluginGuest()) {
-      RenderWidgetHostViewMac* rwhv_mac = static_cast<RenderWidgetHostViewMac*>(
-          contents->GetRenderWidgetHostView());
+    auto* rwhv_base = static_cast<RenderWidgetHostViewBase*>(
+        contents->GetRenderWidgetHostView());
+    if (rwhv_base && !rwhv_base->IsRenderWidgetHostViewChildFrame()) {
+      auto* rwhv_mac = static_cast<RenderWidgetHostViewMac*>(rwhv_base);
       if (rwhv_mac->GetInProcessNSView() == object)
         return rwhv_mac;
     }