Don't call ill-balanced removeStyleSheetCandidateNode()

There are possibilities where removeStyleSheetCandidateNode()
is called without addStyleSheetCandidateNode(). We should prevent this
because StyleEngine assumes that its call is balanced.

This change add a flag to ensure it being balanced.

BUG=353525
R=ojan@chromium.org, eseidel@chromium.org
TEST=style-onload-remove-crash.html

Review URL: https://codereview.chromium.org/203253006

git-svn-id: svn://svn.chromium.org/blink/trunk@169691 bbb929c8-8fbe-4397-9dbb-9b2b20218538