Handle missing glyphs when adjusting spacing in SimpleShaper

Handle the case where the space glyph is used instead of a missing zero-
width-space, soft-hypen or non-printable space glyph when adjusting the
spacing in SimpleShaper::advanceInternal.

TEST=fast/text/soft-hyphen-simple-text.html
BUG=422050
R=pdr@chromium.org

Review URL: https://codereview.chromium.org/648293003

git-svn-id: svn://svn.chromium.org/blink/trunk@183685 bbb929c8-8fbe-4397-9dbb-9b2b20218538