Move more string_util functions to base namespace.

extensions/browser/api/vpn_provider/vpn_provider_api.cc

Index: extensions/browser/api/vpn_provider/vpn_provider_api.cc
diff --git a/extensions/browser/api/vpn_provider/vpn_provider_api.cc b/extensions/browser/api/vpn_provider/vpn_provider_api.cc
index f38a1e275153721f48958f5ee3811d1ff66c5458..fb82aab15def5845f8b0ee5c4e7b33db567dc93b 100644
--- a/extensions/browser/api/vpn_provider/vpn_provider_api.cc
+++ b/extensions/browser/api/vpn_provider/vpn_provider_api.cc
@@ -32,7 +32,7 @@ bool CheckIPCIDRSanity(const std::string& value, bool cidr, bool ipv6) {
   int counter = 0;
 
   for (const auto& elem : value) {
-    if (IsAsciiDigit(elem)) {
+    if (base::IsAsciiDigit(elem)) {
       counter++;
       continue;
     }
@@ -53,7 +53,7 @@ bool CheckIPCIDRSanity(const std::string& value, bool cidr, bool ipv6) {
       if (!colon)
         return false;
       colon--;
-    } else if (!hex_allowed || !IsHexDigit(elem)) {
+    } else if (!hex_allowed || !base::IsHexDigit(elem)) {
       return false;
     } else {
       counter++;