Change getFallbackFamily to take a UChar32

getFallbackFamily is used by both FontCacheSkiaWin and UniscribeHelper
however each uses the method in slightly different ways:

- FontCacheSkiaWin calls it for a single character, converted from
  UCS-4 to USC-2. That value is then passed to the getFallbackFamily
  method where it is converted back to a USC-4 character.

- UniscribeHelper calls it for an array of characters where it depends
  on getFallbackFamily to select the first one specific to a certain
  script.

Change getFallbackFamily to take a single UCS-4 character (instead of
an array of USC-2 ones), avoiding unnecessary conversions and add a
separate method that contains the first specific character and USC-2
to USC-4 conversion logic.

BUG=311769
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/46023002

git-svn-id: svn://svn.chromium.org/blink/trunk@160646 bbb929c8-8fbe-4397-9dbb-9b2b20218538