As of https://bugs.webkit.org/show_bug.cgi?id=135682, the long mouse press is behind a runtime switch that defaults to off. Therefore it is possible to enable the LONG_MOUSE_PRESS feature flag without affecting the behavior of clients who choose not to use the long mouse press.
Created attachment 236218 [details] Patch
ENABLE_LONG_MOUSE_PRESS was removed in the patch for bug #135682, <http://trac.webkit.org/changeset/172503>.