Move fontContainsCharacter method to FontPlatformData

FontCacheSkiaWin and FontCacheSkia both currently have near identical
static fontContainsCharacter functions. The only difference is that the
FontCacheSkiaWin implementation takes an extra font family parameter
however that parameter is never used.

Merge the two implementation and move the resulting one to
FontPlatformData.

R=pdr@chromium.org
BUG=

Review URL: https://codereview.chromium.org/362113002

git-svn-id: svn://svn.chromium.org/blink/trunk@177335 bbb929c8-8fbe-4397-9dbb-9b2b20218538