Teach PasswordAutofillAgent sometimes match prefixes of usernames

components/autofill/core/common/autofill_util.h

Index: components/autofill/core/common/autofill_util.h
diff --git a/components/autofill/core/common/autofill_util.h b/components/autofill/core/common/autofill_util.h
index 6a67f3ef8704ff75be3fe2617000abbd6e7cf3ef..e51d7bd3d4f241e06c579d35fa0c8002829861da 100644
--- a/components/autofill/core/common/autofill_util.h
+++ b/components/autofill/core/common/autofill_util.h
@@ -56,6 +56,12 @@ bool FieldIsSuggestionSubstringStartingOnTokenBoundary(
     const base::string16& field_contents,
     bool case_sensitive);
 
+// Currently, a token for the purposes of this method is defined as {'@'}.
+// Returns true if the |full_string| has a |prefix| as a prefix and the prefix
+// ends on a token.
+bool IsPrefixOfEmailEndingWithAtSign(const base::string16& full_string,
+                                     const base::string16& prefix);
+
 // Finds the first occurrence of a searched substring |field_contents| within
 // the string |suggestion| starting at token boundaries and returns the index to
 // the end of the located substring, or base::string16::npos if the substring is