DevTools: Implement console message logging through an extension

Due to the upcoming split of the "sdk" module, certain types will not be able to access
ConsoleModel. As such, a specific message logging API should be exposed to all modules
to use in place of direct WebInspector.console.log/showErrorMessage calls.

R=sergeyv@chromium.org, yurys@chromium.org, pfeldman, sergeyv

Review URL: https://codereview.chromium.org/298333003

git-svn-id: svn://svn.chromium.org/blink/trunk@175061 bbb929c8-8fbe-4397-9dbb-9b2b20218538