Add NULL check for renderBox::layer() on applying zoom level change

BUG=291293,296164
R=tony@chromium.org
TEST="The repro step on crbug.com/296164 should not crash"

Review URL: https://chromiumcodereview.appspot.com/23526081

git-svn-id: svn://svn.chromium.org/blink/trunk@158238 bbb929c8-8fbe-4397-9dbb-9b2b20218538