Remove bogus if check in ShadowTreeStyleSheetCollection::collectStyleSheets()

Remove bogus if check in ShadowTreeStyleSheetCollection::collectStyleSheets().
The condition will always be true as HTMLStyleElement always has a styleTag
for tag name.

R=esprehn, morrita@chromium.org

Review URL: https://codereview.chromium.org/199893006

git-svn-id: svn://svn.chromium.org/blink/trunk@169713 bbb929c8-8fbe-4397-9dbb-9b2b20218538