Revert "Revert "SkTypes: extract SkTo""

This reverts commit fdcfb8b7c23fbf18f872d2c31d27978235033876.

> Original change's description:
> > SkTypes: extract SkTo
> >
> > Change-Id: I8de790d5013db2105ad885fa2683303d7c250b09
> > Reviewed-on: https://skia-review.googlesource.com/133620
> > Reviewed-by: Mike Klein <mtklein@google.com>

Change-Id: Ida74fbc5c21248a724a5edbf9fae18a33bcb23aa
Reviewed-on: https://skia-review.googlesource.com/134506
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/src/core/SkSafeMath.h b/src/core/SkSafeMath.h
index f54be32..c092272 100644
--- a/src/core/SkSafeMath.h
+++ b/src/core/SkSafeMath.h
@@ -8,6 +8,7 @@
 #ifndef SkSafeMath_DEFINED
 #define SkSafeMath_DEFINED
 
+#include "SkTFitsIn.h"
 #include "SkTypes.h"
 #include <limits>