Move FontFaceSet.{check,load}'s argument default into IDL file

The default value of text parameter was handled in C++ code. Now the IDL
parser supports optional default value syntax, let's use it.

BUG=
TEST=fast/css/font*

Review URL: https://codereview.chromium.org/540223002

git-svn-id: svn://svn.chromium.org/blink/trunk@181441 bbb929c8-8fbe-4397-9dbb-9b2b20218538