Remove shouldUseInputMethod

This method is only used in WebViewImpl so call isContentEditable directly, remove the InputType shouldUseInputMethod methods too.

Review URL: https://codereview.chromium.org/408883003

git-svn-id: svn://svn.chromium.org/blink/trunk@178691 bbb929c8-8fbe-4397-9dbb-9b2b20218538