Allow Desktop Capture API from incognito tabs.

Previously Desktop Capture API allowed requests only from non-incognitor
tabs. The API might be useful in incognito tabs in some scenarios.

R=joi@chromium.org

Review URL: https://codereview.chromium.org/126433003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243706 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243706