Use v8::ExternalOneByteStringResource instead of ExternalAsciiStringResource.

The latter is going to be deprecated soon.

Review URL: https://codereview.chromium.org/642933005

git-svn-id: svn://svn.chromium.org/blink/trunk@183525 bbb929c8-8fbe-4397-9dbb-9b2b20218538