WebFontInfo::familyForChars is only ever called with one or two code points

Just fix it to take a UChar32 instead.

This is patch one of 4 for fixing this.  I have to expose the new API
for Content to consume, then I will fix the code in content, adding
a new UChar32 API for Blink, then remove the old code in Content
then remove the old code in Blink.

Once this is all done I will fix WebFontInfo to be fast
in the single character case (now that the code is clear that's
the only case it needs to support).

U16_GET is documented here:
http://www.icu-project.org/apiref/icu4c/utf16_8h.html#aa354b12f5f755fe734f957cde6d68bbe

BUG=266214
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/20732003

git-svn-id: svn://svn.chromium.org/blink/trunk@155286 bbb929c8-8fbe-4397-9dbb-9b2b20218538