Mongolian fonts need to shape across U+202F NARROW NO-BREAK SPACE
(NNBSP). But if the first font in the fallback chain supports NNBSP,
it would break Mongolian shaping since the text would be broken into
three font runs. By making NNBSP sticky, we make sure Mongolian text
is kept in one font run and is shaped properly.
See http://www.unicode.org/L2/L2017/17036-mongolian-suffix.pdf for
background. The proposed character in the proposal was not accepted
for encoding by the Unicode Techincal Committee, but the document
explains in more detail why this change in needed.
Bug: 34344220
Test: manual
Change-Id: I344a63f383fa5485875603570025eac3c4eb2574
30b9f327d3