Add autofill preview support for <month> input fields

LayoutTests/fast/forms/suggested-value-expected.txt

Index: LayoutTests/fast/forms/suggested-value-expected.txt
diff --git a/LayoutTests/fast/forms/suggested-value-expected.txt b/LayoutTests/fast/forms/suggested-value-expected.txt
index 5bd1f555d18ce50199c73ffd1ea45c4e8997dd2b..78b65422763be92a662131c0c1986fd38df85441 100644
--- a/LayoutTests/fast/forms/suggested-value-expected.txt
+++ b/LayoutTests/fast/forms/suggested-value-expected.txt
@@ -10,6 +10,55 @@ instead of "initial value".
 |     <div>
 |       id="inner-editor"
 |       "suggested value"
+| <input>
+|   id="month"
+|   type="month"
+|   this.value=""
+|   <shadow:root>
+|     <div>
+|       id="date-time-edit"
+|       pseudo="-webkit-datetime-edit"
+|       shadow:pseudoId="-webkit-datetime-edit"
+|       <div>
+|         pseudo="-webkit-datetime-edit-fields-wrapper"
+|         shadow:pseudoId="-webkit-datetime-edit-fields-wrapper"
+|         <span>
+|           aria-help="Month"
+|           aria-valuemax="12"
+|           aria-valuemin="1"
+|           aria-valuenow="1"
+|           aria-valuetext="January"
+|           pseudo="-webkit-datetime-edit-month-field"
+|           role="spinbutton"
+|           shadow:pseudoId="-webkit-datetime-edit-month-field"
+|           "January"
+|         <div>
+|           pseudo="-webkit-datetime-edit-text"
+|           shadow:pseudoId="-webkit-datetime-edit-text"
+|           " "
+|         <span>
+|           aria-help="Year"
+|           aria-valuemax="275760"
+|           aria-valuemin="1"
+|           aria-valuenow="2014"
+|           aria-valuetext="2014"
+|           pseudo="-webkit-datetime-edit-year-field"
+|           role="spinbutton"
+|           shadow:pseudoId="-webkit-datetime-edit-year-field"
+|           "2014"
+|     <div>
+|       id="clear"
+|       pseudo="-webkit-clear-button"
+|       style=""
+|       shadow:pseudoId="-webkit-clear-button"
+|     <div>
+|       id="spin"
+|       pseudo="-webkit-inner-spin-button"
+|       shadow:pseudoId="-webkit-inner-spin-button"
+|     <div>
+|       id="picker"
+|       pseudo="-webkit-calendar-picker-indicator"
+|       shadow:pseudoId="-webkit-calendar-picker-indicator"
 | <textarea>
 |   id="textarea"
 |   this.value="initial value"
@@ -21,5 +70,7 @@ instead of "initial value".
 | "internals.suggestedValue(input): suggested value"
 | "input.selectionStart: 0"
 | "input.selectionEnd: 0"
+| "month.value: "
+| "internals.suggestedValue(month): 2014-01"
 | "textarea.value: initial value"
 | "internals.suggestedValue(textarea): suggested value"