Support Android username-only credentials.

components/autofill/core/common/password_form.h

Index: components/autofill/core/common/password_form.h
diff --git a/components/autofill/core/common/password_form.h b/components/autofill/core/common/password_form.h
index f58222d94cc0f1dcd7d9ba9a856b62dc7ab3711b..aa3b3c1c61d4e851db43893f78f54abee128660b 100644
--- a/components/autofill/core/common/password_form.h
+++ b/components/autofill/core/common/password_form.h
@@ -53,12 +53,14 @@ struct PasswordForm {
   // Enum to differentiate between HTML form based authentication, and dialogs
   // using basic or digest schemes. Default is SCHEME_HTML. Only PasswordForms
   // of the same Scheme will be matched/autofilled against each other.
-  enum Scheme {
+  enum Scheme : int {
     SCHEME_HTML,

[Mike West, 2015/09/29 09:36:09]

Nit: `= 0`? Or does `: int` imply that?
Nit: `unsigned` rather than `int`?

[engedy, 2015/10/08 17:31:01]

Whether the underlying type is fixed or not has nothing to do with the values
that get assigned to each enumerator. §7.2.2 in the standard states that: "If
the first enumerator has no initializer, the value of the corresponding constant
is zero.".
 
I'm not sure if it makes sense to make this unsigned. The corresponding value in
the protobuf is a signed integer anyway.

[vasilii, 2015/10/12 15:12:48]

Acknowledged.

     SCHEME_BASIC,
     SCHEME_DIGEST,
     SCHEME_OTHER,
-    SCHEME_LAST = SCHEME_OTHER
+    SCHEME_USERNAME_ONLY,
+
+    SCHEME_LAST = SCHEME_USERNAME_ONLY
   } scheme;
 
   // During form parsing, Chrome tries to partly understand the type of the form