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, which
is revered by uninitialized member variables of |Font|.

BUG=618237, 622566
TEST=n/a; MSAN will check this

Review-Url: https://codereview.chromium.org/2091633002
Cr-Commit-Position: refs/heads/master@{#401567}