Enable ExternalPopupMenu-related code only on Mac and Android.
- Build external_popup_menu.{cc,h} only on Mac and Android.
- RenderViewImpl and RenderWidget should have external_popup_menu_-related
code only on Mac and Android.
- Define ViewHostMsg_ShowPopup and ViewHostMsg_HidePopup only on Mac and
Android.
- Add default implementations to WebContentsViewDelegateView::
ShowPopupMenu and HidePopupMenu to simplify the code.
R=tsepez@chromium.org, avi@chromium.org
BUG=
Review URL: https://codereview.chromium.org/
209723003
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@259210 0039d316-1c4b-4281-b951-d872f2087c98