Add in a command-line switch for ErrorConsole ('enable-error-console').
Add in duplicate-error detection, so that multiple copies of the same error aren't both stored.
Add in a check so that the ErrorConsole only stores errors if the user is in Developer Mode.
Rename ManifestParsingError to ManifestError, JavascriptRuntimeError to RuntimeError
BUG=21734
Review URL: https://chromiumcodereview.appspot.com/23311011
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@220108 0039d316-1c4b-4281-b951-d872f2087c98