Make default constructor of Font class to initialize all member variables
This patch makes default constructor of |Font| class to initialize
|m_canShapeWordByWord| and |m_shapeWordByWordComputed| member variables to
make MSAN happy.
This patch is a preparation of re-landing http://crrev.com/2082893005
is revered by uninitialized member variables of |Font|.
TEST=n/a; MSAN will check this