WebView: add an API method to control web contents debugging

This adds static (as this is for the whole process) method
for controlling the web debugging state.

Please note that the implementation only makes sense for
Chromium WebView, and it's a no-op in WebViewClassic.

BUG=8691809

Change-Id: I815b4d69ad9ee814ca4cec1da81490b63ba2da68