Fix a crash when an extension with active declarativeContent actions is unloaded.

The crash was a DCHECK and NULL dereference, so not a security issue.

BUG=304373

Review URL: https://codereview.chromium.org/26570004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227800 0039d316-1c4b-4281-b951-d872f2087c98
Rename cast_unittest to cast_unittests

*_unittests is the common name for all other unit test executables.

TBR=hubbe@chromium.org

Review URL: https://codereview.chromium.org/26601006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227801 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227801