StyleElement::removedFromDocument should invoke removePendingSheet if the sheet is loading.

BUG=345114
TEST=fast/css/remove-loading-style.html

Review URL: https://codereview.chromium.org/173463002

git-svn-id: svn://svn.chromium.org/blink/trunk@167861 bbb929c8-8fbe-4397-9dbb-9b2b20218538