Handle case when XSL stylesheet node is removed during load

      When a XSL stylesheet node is removed from document while its content
is being loaded, the current code doesn't handle the case to clear the resource,
while when the node is added again the new resource is created again.

This causes reference count (StyleEngine::m_pendingStyleSheets) leakage.

BUG=427249
TEST=minimized clusterfuzz test

Review URL: https://codereview.chromium.org/722093002

git-svn-id: svn://svn.chromium.org/blink/trunk@185504 bbb929c8-8fbe-4397-9dbb-9b2b20218538