Flip the flag for Carbon plugin interposing to enable

Disable Carbon plugin interposing by default. The theory is that there
are probably not any must-not-break plugins still relying on the Carbon
calls, so this is likely no longer necessary. Keeping the flag, but
flipped, will allow for easy testing if any bugs do show up.

BUG=157130

Review URL: https://codereview.chromium.org/110853009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241990 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241990