blink::BasicShapeInset::operator== crashes

We need to null-check m_shape before calling operator== on it.

R=esprehn@chromium.org
BUG=406990

Review URL: https://codereview.chromium.org/534783002

git-svn-id: svn://svn.chromium.org/blink/trunk@181272 bbb929c8-8fbe-4397-9dbb-9b2b20218538