Guard against reading uninitialized memory in HarfBuzzShaper

Add guard and assert in setGlyphPositionsForHarfBuzzRun to help track
down cause of Mac crasher.

BUG=420551
R=leviw@chromium.org

Review URL: https://codereview.chromium.org/635613002

git-svn-id: svn://svn.chromium.org/blink/trunk@183300 bbb929c8-8fbe-4397-9dbb-9b2b20218538