Bail out from shaping if glyphs are missing

Handle missing glyphs in HarfBuzzShaper gracefully instead of crashing.

BUG=337886
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/171423016

git-svn-id: svn://svn.chromium.org/blink/trunk@167576 bbb929c8-8fbe-4397-9dbb-9b2b20218538