DevTools: Delay loading of node attributes upon DOM.inlineStyleInvalidated by 20ms

This should avoid too quick updates of the Elements panel, which result in CPU hogging.

R=aandrey, eustas, vsevik
BUG=328725

Review URL: https://codereview.chromium.org/118233003

git-svn-id: svn://svn.chromium.org/blink/trunk@164164 bbb929c8-8fbe-4397-9dbb-9b2b20218538