Use Dequeue::removeLast() in StyleResolver::addToStyleSharingList()

Use Dequeue::removeLast() in StyleResolver::addToStyleSharingList() instead of
Dequeue::remove(--end()) to remove the last element in the Dequeue. This
increases the code readability a bit and removeLast() implementation is a bit
simpler.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/395933002

git-svn-id: svn://svn.chromium.org/blink/trunk@178200 bbb929c8-8fbe-4397-9dbb-9b2b20218538