Whitelist the Files app to use the experimental parts of webView.

This patch fixes the Sharing dialog in Files app, which uses webRequest over webView. The experimental part is available on dev channel by default. Therefore, the Sharing dialog stopped working on the beta channel. This patch addresses this issue by whitelisting Files app in the same way as other extensions are whitelisted.

TEST=Tested manually on M30 branch ToT. Confirmed, that it works after applying this patch on the stable channel.
BUG=288783, 264852
R=asargent@chromium.org

Review URL: https://codereview.chromium.org/24047012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225182 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225182