Remove unnecessary work from Node finalizer

By the time we run the Node destructor, we should have already destroyed the
renderer for the node. This CL changes the Node finalizer from handling the
error case gracefully to crashing. Crashing in this case will let us get
feedback from fuzzers and the field as to whether this invariant actually
holds.

The code was introduced in http://src.chromium.org/viewvc/blink?view=revision&revision=798
in a merge from KDE.

R=esprehn@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23461043

git-svn-id: svn://svn.chromium.org/blink/trunk@157536 bbb929c8-8fbe-4397-9dbb-9b2b20218538