Use following font data and script for heading zero-width spaces while collecting candidate runs.

Use following font data and script for heading zero-width spaces while collecting candidate runs.

Collecting a candidate run of heading zero-width spaces in a text run
makes unintended fallback fonts loading and this can effect box height
computation.

With this patch, HarfBuzzShaper will skip to collect a candidate run
for the heading zero-width spaces and use the following font data and
script if there is following runs for the normalized buffer.

BUG=