Set HarfBuzz buffer flags for text boundaries

If the run is the start and/or end of the whole text string being laid
out, set corresponding HarfBuzz buffer flags as it will help HarfBuzz
e.g. place a stray combining at the beginning of a paragraph on a dotted
circle.

Change-Id: I05c9fa46b251a2ce4e716da33fb6edda680d2dd1
Minor

Change-Id: Ic2182ffc76d63b966fc381dadb59af344dda945e