DevTools: NetworkPanel: support "selfNodeHeight" contract.

Supply row height corresponding to current data grid style.

BUG=316092

Review URL: https://codereview.chromium.org/420183003

git-svn-id: svn://svn.chromium.org/blink/trunk@179115 bbb929c8-8fbe-4397-9dbb-9b2b20218538