[Autofill] Add FormEvent logging for "will submit form".

components/autofill/core/browser/autofill_metrics.h

Index: components/autofill/core/browser/autofill_metrics.h
diff --git a/components/autofill/core/browser/autofill_metrics.h b/components/autofill/core/browser/autofill_metrics.h
index 78540dc4127fcf47760472d4b5302386781ac989..b7cff968d914d958d1e61589fe0fee1f5a353e47 100644
--- a/components/autofill/core/browser/autofill_metrics.h
+++ b/components/autofill/core/browser/autofill_metrics.h
@@ -269,6 +269,10 @@ class AutofillMetrics {
     FORM_EVENT_LOCAL_SUGGESTION_FILLED_ONCE,
     FORM_EVENT_SERVER_SUGGESTION_FILLED_ONCE,
     FORM_EVENT_MASKED_SERVER_CARD_SUGGESTION_FILLED_ONCE,
+    // An autofillable form is about to be submitted. If the submission is not
+    // interrupted by JavaScript, the "form submitted" events below will also be
+    // logged.
+    FORM_EVENT_WILL_SUBMIT,

[Ilya Sherman, 2015/03/18 22:42:10]

Why are you adding this event in the middle of the list?  I believe that this
enumeration backs an enumerated UMA histogram, right?

[Mathieu, 2015/03/23 15:24:02]

Done. Yes of course.

     // A form was submitted. Depending on the user filling a local, server,
     // masked server card or no suggestion one of the following will be
     // triggered. Only one of the following four will be triggered per page
@@ -281,6 +285,7 @@ class AutofillMetrics {
     FORM_EVENT_MASKED_SERVER_CARD_SUGGESTION_SELECTED,
     // Same as above but only triggered once per page load.
     FORM_EVENT_MASKED_SERVER_CARD_SUGGESTION_SELECTED_ONCE,
+
     NUM_FORM_EVENTS,
   };
 
@@ -494,6 +499,8 @@ class AutofillMetrics {
 
     void OnDidFillSuggestion(const AutofillProfile& profile);
 
+    void OnWillSubmitForm();
+
     void OnDidSubmitForm();
 
    private: