Add static to global nullopt symbol.

This should hopefully make the code safer in C++14 by not violating
the ODR.

(bungeman@: "I think in C++14 we will in fact get multiple definitions
of this symbol, though in C++17 we won't. Maybe we should mark this as
'static' so each TU gets its own zero byte copy in C++14 but in C++17
the 'static' will also be implicitly 'static inline' and we'll only get
one?)

Change-Id: I3570421fcf2be314e1baccff0783ded4a1078666
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/427037
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
1 file changed