Distribute widths evenly among code points in SynthesizeGraphemeWidths

Previously we did not consider the possibility of a typographic unit
containing multiple surrogate pairs, and could as an effect "lose" part
of the width.

Distribute the width evenly among code points instead to at least get
some sort of consistency.

Bug: 879361
Change-Id: I51a4e6c574926215a087b0bc824ac90d580ff169
Reviewed-on: https://chromium-review.googlesource.com/1199423
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#588183}
2 files changed