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This allows KeySystems to call GetContentClient()->AddKeySystems() so we can
move key system information to chrome/. The new ContentClient API is defined and
called, but we still rely on key_systems_info.cc.

BUG=224793
TEST=Existing content_unittests and content_browsertests pass.
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/23484019

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222074 0039d316-1c4b-4281-b951-d872f2087c98