Add namespace and re-arrange PDFium's local copy of chromium /base.

third_party/base/numerics/safe_math_impl.h

Index: third_party/base/numerics/safe_math_impl.h
diff --git a/third_party/numerics/safe_math_impl.h b/third_party/base/numerics/safe_math_impl.h
similarity index 99%
rename from third_party/numerics/safe_math_impl.h
rename to third_party/base/numerics/safe_math_impl.h
index 4bf59e64e07c6ce6edde7deee2e2014bc623749c..8f1543d308f893533dc0b20da21ea70f9c13b267 100644
--- a/third_party/numerics/safe_math_impl.h
+++ b/third_party/base/numerics/safe_math_impl.h
@@ -15,6 +15,7 @@
 #include "../template_util.h"
 #include "safe_conversions.h"
 
+namespace third_party {
 namespace base {
 namespace internal {
 
@@ -498,5 +499,6 @@ struct IsIntegerArithmeticSafe {
 
 }  // namespace internal
 }  // namespace base
+}  // namespace third_party
 
 #endif  // SAFE_MATH_IMPL_H_