DevTools: [SSP] fix design glitches in "Insert New Rule"

The patch does the following:
- adds tooltip for the "Insert New Rule" button
- repositions button via absolute positioning
- does not show "via inspector" title in "Insert New Rule" if it's going
to be added in actual stylesheet.

R=pfeldman, vsevik
BUG=402461

Review URL: https://codereview.chromium.org/468973002

git-svn-id: svn://svn.chromium.org/blink/trunk@180204 bbb929c8-8fbe-4397-9dbb-9b2b20218538