Clean up a few IWYU issues

Spun off from the SkTFitsIn CL.

Change-Id: I686d680df6a36ebc02db3847ad5e2cedcbcd67ef
Reviewed-on: https://skia-review.googlesource.com/134083
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/src/core/SkSafeMath.h b/src/core/SkSafeMath.h
index 0bc0fbf..f54be32 100644
--- a/src/core/SkSafeMath.h
+++ b/src/core/SkSafeMath.h
@@ -9,6 +9,7 @@
 #define SkSafeMath_DEFINED
 
 #include "SkTypes.h"
+#include <limits>
 
 // SkSafeMath always check that a series of operations do not overflow.
 // This must be correct for all platforms, because this is a check for safety at runtime.