DevTools: Do not re-create CSSWorkspaceBinding TargetInfo on MainFrameNavigated

R=vsevik

Review URL: https://codereview.chromium.org/429593006

git-svn-id: svn://svn.chromium.org/blink/trunk@179168 bbb929c8-8fbe-4397-9dbb-9b2b20218538