DevTools: Remove accessor to unexisting property.

JSCompiler finds this inconsistency with my ongoing patch applied (that removes type getter in NetworkRequest)

BUG=

Review URL: https://codereview.chromium.org/670463003

git-svn-id: svn://svn.chromium.org/blink/trunk@184068 bbb929c8-8fbe-4397-9dbb-9b2b20218538