Remove unused instance tree related member variable

The plan is to remove the instance tree. Independent of that, this
member variable seems to be set but not used.

BUG=313438

Review URL: https://codereview.chromium.org/254803008

git-svn-id: svn://svn.chromium.org/blink/trunk@172707 bbb929c8-8fbe-4397-9dbb-9b2b20218538