Ship "formdata" event.

Intent-to-ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/ONvzLM5fPvk/FZDswIOpAwAJ

Bug: 825684
Change-Id: I5b1108b72a345e2fb59596e58f7e13d6b772efe7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635111Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665054}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -638,7 +638,7 @@
     },
     {
       name: "FormDataEvent",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "FractionalMouseEvent",

third_party/blink/web_tests/virtual/stable/webexposed/element-instance-property-listing-expected.txt

@@ -142,6 +142,7 @@ namespace http://www.w3.org/1999/xhtml
     property onended
     property onerror
     property onfocus
+    property onformdata
     property onfullscreenchange
     property onfullscreenerror
     property ongotpointercapture
@@ -1235,6 +1236,7 @@ namespace http://www.w3.org/2000/svg
     property onended
     property onerror
     property onfocus
+    property onformdata
     property onfullscreenchange
     property onfullscreenerror
     property ongotpointercapture

third_party/blink/web_tests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -1294,6 +1294,7 @@ interface Document : Node
     getter onended
     getter onerror
     getter onfocus
+    getter onformdata
     getter onfreeze
     getter onfullscreenchange
     getter onfullscreenerror
@@ -1476,6 +1477,7 @@ interface Document : Node
     setter onended
     setter onerror
     setter onfocus
+    setter onformdata
     setter onfreeze
     setter onfullscreenchange
     setter onfullscreenerror
@@ -1860,6 +1862,10 @@ interface FormData
     method keys
     method set
     method values
+interface FormDataEvent : Event
+    attribute @@toStringTag
+    getter formData
+    method constructor
 interface GainNode : AudioNode
     attribute @@toStringTag
     getter gain
@@ -2219,6 +2225,7 @@ interface HTMLElement : Element
     getter onended
     getter onerror
     getter onfocus
+    getter onformdata
     getter ongotpointercapture
     getter oninput
     getter oninvalid
@@ -2319,6 +2326,7 @@ interface HTMLElement : Element
     setter onended
     setter onerror
     setter onfocus
+    setter onformdata
     setter ongotpointercapture
     setter oninput
     setter oninvalid
@@ -5468,6 +5476,7 @@ interface SVGElement : Element
     getter onended
     getter onerror
     getter onfocus
+    getter onformdata
     getter ongotpointercapture
     getter oninput
     getter oninvalid
@@ -5555,6 +5564,7 @@ interface SVGElement : Element
     setter onended
     setter onerror
     setter onfocus
+    setter onformdata
     setter ongotpointercapture
     setter oninput
     setter oninvalid
@@ -8854,6 +8864,7 @@ interface webkitURL
     getter onended
     getter onerror
     getter onfocus
+    getter onformdata
     getter ongotpointercapture
     getter onhashchange
     getter oninput
@@ -9038,6 +9049,7 @@ interface webkitURL
     setter onended
     setter onerror
     setter onfocus
+    setter onformdata
     setter ongotpointercapture
     setter onhashchange
     setter oninput