Adding Info message level.

This code extracted from https://codereview.chromium.org/25003002/

It needs to be landed in advance to fix chromium code
RenderViewImpl::didAddMessageToConsole before landing main change.

BUG=160545

Review URL: https://codereview.chromium.org/34613004

git-svn-id: svn://svn.chromium.org/blink/trunk@160460 bbb929c8-8fbe-4397-9dbb-9b2b20218538