ASSERT that typeface is set in SimpleFontData::platformInit

Add ASSERT to SimpleFontData::platformInit ensuring that a typeface has
been specified as it is required by getTableSize, getUnitsPerEm etc.

R=dglazkov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/69543008

git-svn-id: svn://svn.chromium.org/blink/trunk@161863 bbb929c8-8fbe-4397-9dbb-9b2b20218538