KeyboardEvent.code is always empty on Android (#25644)

* KeyboardEvent.code is always empty on Android

* Apply suggestions from code review

Author: caugner

api/KeyboardEvent.json

@@ -174,12 +174,20 @@
             "chrome": {
               "version_added": "48"
             },
-            "chrome_android": "mirror",
+            "chrome_android": {
+              "version_added": "48",
+              "partial_implementation": true,
+              "notes": "The value is always empty."
+            },
             "edge": "mirror",
             "firefox": {
               "version_added": "38"
             },
-            "firefox_android": "mirror",
+            "firefox_android": {
+              "version_added": "38",
+              "partial_implementation": true,
+              "notes": "The value is always empty."
+            },
             "ie": {
               "version_added": false
             },