Check transform() before dereferencing it.

hasTransform() does not mean transform() is non-null.

BUG=357315

Review URL: https://codereview.chromium.org/217713003

git-svn-id: svn://svn.chromium.org/blink/trunk@170400 bbb929c8-8fbe-4397-9dbb-9b2b20218538