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

components/autofill/content/renderer/form_autofill_util.cc

Index: components/autofill/content/renderer/form_autofill_util.cc
diff --git a/components/autofill/content/renderer/form_autofill_util.cc b/components/autofill/content/renderer/form_autofill_util.cc
index c9ebc62c3996f80d2205c16dd60be43d3a05382f..1a5d474d9dccff923927aa573cf25158fb4eaee9 100644
--- a/components/autofill/content/renderer/form_autofill_util.cc
+++ b/components/autofill/content/renderer/form_autofill_util.cc
@@ -16,6 +16,7 @@
 #include "components/autofill/core/common/autofill_data_validation.h"
 #include "components/autofill/core/common/autofill_regexes.h"
 #include "components/autofill/core/common/autofill_switches.h"
+#include "components/autofill/core/common/autofill_util.h"
 #include "components/autofill/core/common/form_data.h"
 #include "components/autofill/core/common/form_field_data.h"
 #include "third_party/WebKit/public/platform/WebString.h"
@@ -871,9 +872,15 @@ void PreviewFormField(const FormFieldData& data,
   if (is_initiating_node &&
       (IsTextInput(input_element) || IsTextAreaElement(*field))) {
     // Select the part of the text that the user didn't type.
-    int start = field->value().length();
-    int end = field->suggestedValue().length();
-    field->setSelectionRange(start, end);
+    if (IsFeatureSubstringMatchEnabled()) {
+      size_t start = autofill::GetTextSelectionStart(field->suggestedValue(),
+                                                     field->value());
+      if (start != base::string16::npos)
+        field->setSelectionRange(start, field->suggestedValue().length());
+    } else {
+      field->setSelectionRange(field->value().length(),
+                               field->suggestedValue().length());
+    }
   }
 }