FontPlatformDataHarfBuzz incorrectly shares HarfBuzzFace

The copy construcors FontPlatformDataHarfBuzz incorrectly copies the
m_harfBuzzFace pointer. As the copy constructs allows certain
properties to be overridden this is incorrect.

Furthermore HarfBuzzFace implements caching based on uniqueId so the
HarfBuzzFace objects will be shared when safe to do so regardless.

BUG=306224
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/26910003

git-svn-id: svn://svn.chromium.org/blink/trunk@159391 bbb929c8-8fbe-4397-9dbb-9b2b20218538