rAc: make requestAutocomplete() return a promise.

LayoutTests/fast/forms/form-request-autocomplete-expected.txt

Index: LayoutTests/fast/forms/form-request-autocomplete-expected.txt
diff --git a/LayoutTests/fast/forms/form-request-autocomplete-expected.txt b/LayoutTests/fast/forms/form-request-autocomplete-expected.txt
index e38661f8bcd9f7fe428faae58faeed3ccb3a5d08..a4debafffbcbbf7fd55e50b77967e557bec5fa28 100644
--- a/LayoutTests/fast/forms/form-request-autocomplete-expected.txt
+++ b/LayoutTests/fast/forms/form-request-autocomplete-expected.txt
@@ -1,20 +1,30 @@
 CONSOLE MESSAGE: requestAutocomplete: form autocomplete attribute is set to off.
 CONSOLE MESSAGE: requestAutocomplete: form autocomplete attribute is set to off.
 CONSOLE MESSAGE: requestAutocomplete: form autocomplete attribute is set to off.
+CONSOLE MESSAGE: requestAutocomplete: form autocomplete attribute is set to off.
 CONSOLE MESSAGE: requestAutocomplete: must be called in response to a user gesture.
 HTMLFormElement#requestAutocomplete and associated events
 
 For this test to pass, you should see all PASSED below.
 
+DEBUG checkDynamicAttributes running
 PASS found enumerable properties on HTMLFormElement
+DEBUG checkParsedAttributes running
 PASS found enumerable properties on HTMLFormElement
-PASS found enumerable properties on HTMLFormElement
+DEBUG checkEventsBubble running
+DEBUG checkPromiseRejects running
+PASS requestAutocomplete() returns a Promise
+DEBUG checkNonUserGesture running
+DEBUG finished runs tests, waiting for results
+DEBUG handling error from checkDynamicAttributes
 PASS event is an AutocompleteErrorEvent
 PASS got expected reason: disabled
+DEBUG handling error from checkNonUserGesture
 PASS event is an AutocompleteErrorEvent
 PASS got expected reason: disabled
 PASS events bubbled as expected
-PASS got expected number of error events (4)
+PASS Promise.catch() fired
+PASS got expected number of error events (5)
 PASS successfullyParsed is true
 
 TEST COMPLETE