Remove unused m_isPlatformFont variable.

This is no longer used, and we can remove it,
as well as the respective plumbing.

R=eae,ksakamoto@chromium.org
BUG=315783

Review URL: https://codereview.chromium.org/138953004

git-svn-id: svn://svn.chromium.org/blink/trunk@165708 bbb929c8-8fbe-4397-9dbb-9b2b20218538