[Autofill/Autocomplete Feature] Substring matching instead of prefix matching.

components/autofill/content/renderer/password_autofill_agent.cc

Index: components/autofill/content/renderer/password_autofill_agent.cc
diff --git a/components/autofill/content/renderer/password_autofill_agent.cc b/components/autofill/content/renderer/password_autofill_agent.cc
index f0dcefd80475482fc096bf1c14839fd18cab75d4..727b515bf6d699480732d09f408ac11f820e69ec 100644
--- a/components/autofill/content/renderer/password_autofill_agent.cc
+++ b/components/autofill/content/renderer/password_autofill_agent.cc
@@ -4,6 +4,8 @@
 
 #include "components/autofill/content/renderer/password_autofill_agent.h"
 
+#include <algorithm>
+
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/memory/scoped_ptr.h"
@@ -778,10 +780,10 @@ bool PasswordAutofillAgent::FillSuggestion(
   return true;
 }
 
-bool PasswordAutofillAgent::PreviewSuggestion(
-    const blink::WebNode& node,
-    const blink::WebString& username,
-    const blink::WebString& password) {
+bool PasswordAutofillAgent::PreviewSuggestion(const blink::WebNode& node,
+                                              const blink::WebString& username,
+                                              const blink::WebString& password,
+                                              int match_start) {
   blink::WebInputElement username_element;
   PasswordInfo* password_info;
 
@@ -792,7 +794,8 @@ bool PasswordAutofillAgent::PreviewSuggestion(
   }
 
   was_username_autofilled_ = username_element.isAutofilled();
-  username_selection_start_ = username_element.selectionStart();
+  username_selection_start_ =
+      std::min(match_start, username_element.maxLength());
   username_element.setSuggestedValue(username);
   username_element.setAutofilled(true);
   username_element.setSelectionRange(