Add a flag to always disable script injection for accessibility.

R=dmazzoni@chromium.org
BUG=none

Review URL: https://codereview.chromium.org/103133005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238857 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238857