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 98%
rename from third_party/numerics/safe_math_impl.h
rename to third_party/base/numerics/safe_math_impl.h
index 4bf59e64e07c6ce6edde7deee2e2014bc623749c..f9a4a715700ad00bd7dec0d77f7fe8aa6de84bdd 100644
--- a/third_party/numerics/safe_math_impl.h
+++ b/third_party/base/numerics/safe_math_impl.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef SAFE_MATH_IMPL_H_
-#define SAFE_MATH_IMPL_H_
+#ifndef PDFIUM_THIRD_PARTY_SAFE_MATH_IMPL_H_
+#define PDFIUM_THIRD_PARTY_SAFE_MATH_IMPL_H_
 
 #include <stdint.h>
 
@@ -15,6 +15,7 @@
 #include "../template_util.h"
 #include "safe_conversions.h"
 
+namespace pdfium {
 namespace base {
 namespace internal {
 
@@ -498,5 +499,6 @@ struct IsIntegerArithmeticSafe {
 
 }  // namespace internal
 }  // namespace base
+}  // namespace pdfium
 
-#endif  // SAFE_MATH_IMPL_H_
+#endif  // PDFIUM_THIRD_PARTY_SAFE_MATH_IMPL_H_