DevTools: NetworkPanel: do not reset linkifier until node is alive.

This patch is a preparation to butter-smooth viewport in NetworkPanel.

BUG=

Review URL: https://codereview.chromium.org/398763002

git-svn-id: svn://svn.chromium.org/blink/trunk@178336 bbb929c8-8fbe-4397-9dbb-9b2b20218538