Add WebRuntimeFeatures::enableInputModeAttribute

Chromium will use this function to enable inputmode attribute of <input> or <text> Element.

BUG=244688

Review URL: https://chromiumcodereview.appspot.com/23112014

git-svn-id: svn://svn.chromium.org/blink/trunk@156548 bbb929c8-8fbe-4397-9dbb-9b2b20218538