Remove obsolete comment in ShadowRoot.h
Remove obsolete comment in ShadowRoot.h

The comment links to
https://bugs.webkit.org/show_bug.cgi?id=77503
which is already resolved.

Multiple shadow roots still remain for a while and
the comment is valid about that some elements doesn't
allow shadow roots on them, which are tracked at
https://github.com/w3c/webcomponents/issues/102
https://code.google.com/p/chromium/issues/detail?id=234020

Review URL: https://codereview.chromium.org/1156133008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@196554 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk