[KeyboardMap] Enable navigator.keyboard.getLayoutMap() on Stable.

Launch bug (w/ approvals): crbug.com/831743

Intent 2 ship: https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/V2xR7cucnOA

Bug: 832811, 831743
Change-Id: I24d2f49e82b5afe1dd668dc57e0ceb98af36e205
Reviewed-on: https://chromium-review.googlesource.com/1141104
Reviewed-by: Alex Russell <slightlyoff@chromium.org>
Commit-Queue: Gary Kacmarcik <garykac@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#576796}(cherry picked from commit 4fcc638)
Reviewed-on: https://chromium-review.googlesource.com/1147040
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Cr-Commit-Position: refs/branch-heads/3497@{#16}
Cr-Branched-From: 271eaf5-refs/heads/master@{#576753}

third_party/WebKit/LayoutTests/virtual/stable/webexposed/global-interface-listing-expected.txt

@@ -3588,6 +3588,7 @@ interface IntersectionObserverEntry
 interface Keyboard
     attribute @@toStringTag
     method constructor
+    method getLayoutMap
     method lock
     method unlock
 interface KeyboardEvent : UIEvent
@@ -3610,6 +3611,17 @@ interface KeyboardEvent : UIEvent
     method constructor
     method getModifierState
     method initKeyboardEvent
+interface KeyboardLayoutMap
+    attribute @@toStringTag
+    getter size
+    method @@iterator
+    method constructor
+    method entries
+    method forEach
+    method get
+    method has
+    method keys
+    method values
 interface LinearAccelerationSensor : Accelerometer
     attribute @@toStringTag
     method constructor

third_party/blink/renderer/modules/keyboard/keyboard.idl

@@ -17,7 +17,6 @@
 
     // Keyboard Map specification: https://wicg.github.io/keyboard-map/
     [CallWith=ScriptState,
-     RuntimeEnabled=KeyboardMap,
      MeasureAs=KeyboardApiGetLayoutMap
     ] Promise<KeyboardLayoutMap> getLayoutMap();
 };

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -636,7 +636,7 @@
     },
     {
       name: "KeyboardMap",
-      status: "experimental",
+      status: "stable",
     },
     {
       name: "LangAttributeAwareFormControlUI",