webos-preferred-providers: Add VIRTUAL-RUNTIME_browser_fonts

:Release Notes:
Allow the package that supplies the browser fonts to be selected by
the distro via the VIRTUAL-RUNTIME_browser_fonts setting. It's set to
isis-fonts in Open webOS.

:Detailed Notes:

:Testing Performed:
"make webos-image" succeeded.

:QA Notes:

:Issues Addressed:
[GF-11105] Allow the package that supplies the browser fonts to be
           selected by the distro

Open-webOS-DCO-1.0-Signed-off-by: Herb Kuta <herb.kuta@lge.com>
Change-Id: I2863c33fbac5848ea35659828a7db0a2a60ab612
Reviewed-on: https://g2g.palm.com/2926
Tested-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Susan Montooth <susan.montooth@lge.com>