An emoji base with an emoji modifier renders as a single glyph and
thus should not be a line break. Current (Unicode 8) logic does
indicate a line break, so we override the results of the ICU line
break iterator. The code references a proposal to improve Unicode
behavior; when that is adopted and we upgrade ICU accordingly, the
special-case code should be deleted, but the tests can remain.

Bug: 27343378
Change-Id: I5de9c53e9a34c503816f9131e3d894e6f7a57d13