Make ConsoleLogHandler use console.error/warn/info.

Deprecate FirebugLogHandler, but keep it there for backwards compatibility.

Fixes issue 6916 and issue 8040

Change-Id: I65c26c010d2e5e2fa96a4fbbffd1229c7a09f5fe
Review-Link: https://gwt-review.googlesource.com/#/c/2060/

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11550 8db76d5a-ed1c-0410-87a9-c151d255dfc7