Use SkFontMgr for webfont construction on windows

Change FontCache to expose the SkFontMgr implementation and use that to
construct webfonts on windows. This way the appropriate font manager
(DirectWrite or GDI) is used for webfonts just as it is for system
fonts.

BUG=25541
R=schenney@chromium.org,pdr@chromium.org

Review URL: https://codereview.chromium.org/113193005

git-svn-id: svn://svn.chromium.org/blink/trunk@163758 bbb929c8-8fbe-4397-9dbb-9b2b20218538