Remove legacy StartsWithASCII function.

components/proximity_auth/cryptauth/fake_secure_message_delegate.cc

Index: components/proximity_auth/cryptauth/fake_secure_message_delegate.cc
diff --git a/components/proximity_auth/cryptauth/fake_secure_message_delegate.cc b/components/proximity_auth/cryptauth/fake_secure_message_delegate.cc
index d05361c2d1ae50e65bbfcae06519a9974759d82b..4251e1991b06d9b2e255125c5adf8eae6e701e0e 100644
--- a/components/proximity_auth/cryptauth/fake_secure_message_delegate.cc
+++ b/components/proximity_auth/cryptauth/fake_secure_message_delegate.cc
@@ -76,7 +76,8 @@ bool Verify(const std::string& signature,
     signing_key = key;
   } else {
     std::string prefix = kPrivateKeyPrefix;
-    bool is_private_key = base::StartsWithASCII(key, prefix, true);
+    bool is_private_key =
+        base::StartsWith(key, prefix, base::CompareCase::SENSITIVE);
     signing_key = is_private_key ? key.substr(prefix.size()) : prefix + key;
   }
 
@@ -113,7 +114,7 @@ void FakeSecureMessageDelegate::DeriveKey(const std::string& private_key,
   // private key so it is equal to its corresponding public key.
   std::string prefix(kPrivateKeyPrefix);
   std::string normalized_private_key =
-      base::StartsWithASCII(private_key, prefix, true)
+      base::StartsWith(private_key, prefix, base::CompareCase::SENSITIVE)
           ? private_key.substr(prefix.size())
           : private_key;