DevTools: Remove console last result $_ on console clear.

BUG=398336
R=loislo, yurys

Review URL: https://codereview.chromium.org/430503002

git-svn-id: svn://svn.chromium.org/blink/trunk@179179 bbb929c8-8fbe-4397-9dbb-9b2b20218538