Replace base string utils with android::base ones

Test: th
Change-Id: I62c1dfb24ec1dae3cdc5e0a0a93ea2b48e9967e5
diff --git a/common/mock_http_fetcher.cc b/common/mock_http_fetcher.cc
index 1b3cd7d..668d249 100644
--- a/common/mock_http_fetcher.cc
+++ b/common/mock_http_fetcher.cc
@@ -20,11 +20,12 @@
 
 #include <base/bind.h>
 #include <base/logging.h>
-#include <base/strings/string_util.h>
 #include <base/time/time.h>
 #include <brillo/message_loops/message_loop.h>
 #include <gtest/gtest.h>
 
+#include "update_engine/common/utils.h"
+
 // This is a mock implementation of HttpFetcher which is useful for testing.
 
 using brillo::MessageLoop;
@@ -107,11 +108,11 @@
 
 void MockHttpFetcher::SetHeader(const std::string& header_name,
                                 const std::string& header_value) {
-  extra_headers_[base::ToLowerASCII(header_name)] = header_value;
+  extra_headers_[ToLower(header_name)] = header_value;
 }
 
 std::string MockHttpFetcher::GetHeader(const std::string& header_name) const {
-  const auto it = extra_headers_.find(base::ToLowerASCII(header_name));
+  const auto it = extra_headers_.find(ToLower(header_name));
   if (it == extra_headers_.end())
     return "";
   return it->second;