[DevTools] Fixed wrong message count shown after console being cleared.
Fixed testConsoleOnNavigateBack test.

BUG=259664,260341

Review URL: https://chromiumcodereview.appspot.com/19565002

git-svn-id: svn://svn.chromium.org/blink/trunk@154489 bbb929c8-8fbe-4397-9dbb-9b2b20218538