Revert of base: Remove using:: declaration from version.h header.

extensions/common/extension.cc

Index: extensions/common/extension.cc
diff --git a/extensions/common/extension.cc b/extensions/common/extension.cc
index 06544e861a4db36f1386cec9a8acbb21161a9378..b6803857ef917a2aacf79637f471ed28957358dc 100644
--- a/extensions/common/extension.cc
+++ b/extensions/common/extension.cc
@@ -576,7 +576,7 @@
     *error = base::ASCIIToUTF16(errors::kInvalidVersion);
     return false;
   }
-  version_.reset(new base::Version(version_str));
+  version_.reset(new Version(version_str));
   if (!version_->IsValid() || version_->components().size() > 4) {
     *error = base::ASCIIToUTF16(errors::kInvalidVersion);
     return false;