bug#23246: 25.0.92; Local face-remapping-alist doesn't affect tooltips

[Artur Malabarba]

1. Start emacs -q

2. Create a buffer in fundamental-mode and type some text.

3. Evaluate this:

    (setq x-gtk-use-system-tooltips nil)
    (add-text-properties (point-min) (point-max) '(help-echo "OKOK"))
    (face-remap-add-relative 'tooltip '((:foreground "blue")))

4. Tooltips displayed are NOT blue.

5. If, instead, I add an entry to face-remapping-alist without making it
buffer-local, the tooltip does indeed display blue.