Always initialize |m_totalWidth| in HarfBuzzShaper::shape.

      R=leviw@chromium.org
BUG=476647

Review URL: https://codereview.chromium.org/1108663003

git-svn-id: svn://svn.chromium.org/blink/trunk@194541 bbb929c8-8fbe-4397-9dbb-9b2b20218538