Bail out and assert if the glyph to character map is missing

Change HarfBuzzShaper::shapeHarfBuzzRuns to fail early and assert
(instead of crashing) if the runs m_glyphToCharacterIndexes is
unpopulated.
This is a stop-gap measure until we can track down the root cause,
see bug for details.

BUG=337886
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/146693016

git-svn-id: svn://svn.chromium.org/blink/trunk@166096 bbb929c8-8fbe-4397-9dbb-9b2b20218538