Use SkFontMgr for synthetic font styles on windows

Change FontCustomPlatformData::fontPlatformData to use the SkFontMgr
API on Windows for synthetic bold and italic. Without it we'd fall
back on GDI even if DirectWrite is enabled.

BUG=25541
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/141653017

git-svn-id: svn://svn.chromium.org/blink/trunk@165591 bbb929c8-8fbe-4397-9dbb-9b2b20218538