Extend WebFallbackFont by fciId

Required for instantiating the fallback fonts from the sandbox since
this instantiation is not allowed by filename, but only through
Skia's SkFontConfigInterface, where paths are previously kept track
of in the Linux sandbox support process.
Preparation for a Chromium side change which fills fciId.

BUG=285303
R=abarth,eae

Review URL: https://codereview.chromium.org/340073002

git-svn-id: svn://svn.chromium.org/blink/trunk@176494 bbb929c8-8fbe-4397-9dbb-9b2b20218538