DevTools: [Console] add forgotten monospace class to messagesElement

This patch adds missed monospace class to the console view messages
element. This fixes the misaligned anchor links for console messages.

BUG=390483

Review URL: https://codereview.chromium.org/360293002

git-svn-id: svn://svn.chromium.org/blink/trunk@177308 bbb929c8-8fbe-4397-9dbb-9b2b20218538