Encrypted Media: No need to cancel NewKeyCB during tear down.

This is a follow up CL of r251277

Now we invalidate the weak pointers during Decrypting*::Stop(), so all callbacks
are canceled. We don't need to manually cancel the NewKeyCB anymore.

TEST=Media unittests updated.

Review URL: https://codereview.chromium.org/167333004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251876 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251876