Add disable WebRTC encryption tests.

- Broke out copying of the disable encryption command line flag to its own function. Added unit test for it.
- Added browser test.

BUG=342294

Review URL: https://codereview.chromium.org/170623003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252624 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252624