Implement FontPlatformData::isFixedPitch for linux

Implement support for isFixedPitch on linux by using deferring to skia.

R=dglazkov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/224343002

git-svn-id: svn://svn.chromium.org/blink/trunk@170790 bbb929c8-8fbe-4397-9dbb-9b2b20218538