Expose ShadowRoot constructor if "experimental features" flag is enabled.

We currently expose the prefixed constructor (WebKitShadowRoot) but do not
expose the unprefixed one, even if the "experimental web platform features"
flag is enabled. Unprefixed Shadow DOM APIs are available behind a flag, thus
the unprefixed constructor should be available if the runtime flag is enabled.

As we have to provide a constructor under two names, they are defined in
Window.idl for now. When it's ready to remove the prefixed APIs, we will
be able to remove these hacks from Window.idl and get rid of
[NoInterfaceObject] attribute from ShadowRoot interface definition.

No new tests are added; updating existing tests and test results should be
enough.

BUG=242153
R=haraken@chromium.org,tkent@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22370004

git-svn-id: svn://svn.chromium.org/blink/trunk@155648 bbb929c8-8fbe-4397-9dbb-9b2b20218538