Fix crash when accessing Event::path().

(A more elaborate account of the details is found in the bug report.)

BUG=400476
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/516843004

git-svn-id: svn://svn.chromium.org/blink/trunk@181200 bbb929c8-8fbe-4397-9dbb-9b2b20218538