DevTools: fix elements panel node picker code

The code for elements tree outline node selection was executed twice
for every mouse click in case of "node picker" functionality.

To fix this, the patch consumes the mouse click event as the node
was picked.

R=apavlov

Review URL: https://codereview.chromium.org/623723002

git-svn-id: svn://svn.chromium.org/blink/trunk@183152 bbb929c8-8fbe-4397-9dbb-9b2b20218538