Remove InputMode runtime enabled feature.

Now that input mode has shipped in a stable release remove the setting.

BUG=642800

Change-Id: I0aecb0261ba17be7bd911fc2c420d0e3c5dd783f
Reviewed-on: https://chromium-review.googlesource.com/1037983Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555380}

third_party/blink/renderer/core/editing/ime/input_method_controller.cc

@@ -1389,9 +1389,6 @@ int InputMethodController::ComputeWebTextInputNextPreviousFlags() const {
 }
 
 WebTextInputMode InputMethodController::InputModeOfFocusedElement() const {
-  if (!RuntimeEnabledFeatures::InputModeAttributeEnabled())
-    return kWebTextInputModeDefault;
-
   AtomicString mode = GetInputModeAttribute(GetDocument().FocusedElement());
 
   if (mode.IsEmpty())

third_party/blink/renderer/core/html/html_element.idl

@@ -44,7 +44,7 @@ interface HTMLElement : Element {
     // https://html.spec.whatwg.org/#contenteditable
     [CEReactions, CustomElementCallbacks, RaisesException=Setter] attribute DOMString contentEditable;
     [ImplementedAs=isContentEditableForBinding] readonly attribute boolean isContentEditable;
-    [RuntimeEnabled=InputModeAttribute, CEReactions, Reflect, ReflectOnly=("none","text","tel","url","email","numeric","decimal","search")] attribute DOMString inputMode;
+    [CEReactions, Reflect, ReflectOnly=("none","text","tel","url","email","numeric","decimal","search")] attribute DOMString inputMode;
 
     // CSSOM View Module
     // https://drafts.csswg.org/cssom-view/#extensions-to-the-htmlelement-interface

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -602,10 +602,6 @@
       name: "InertAttribute",
       status: "experimental",
     },
-    {
-      name: "InputModeAttribute",
-      status: "stable",
-    },
     {
       name: "InputMultipleFieldsUI",
       status: "stable",