Fix Reset() during waiting-for-a-key in VideoFrameStream.

When the DecryptingDemuxerStream is in the waiting-for-a-key state, we should
reset the DecryptingDemxuerStream in VideoFrameStream::Reset(). Otherwise, the
read callback will never fire and VFS::Reset() never finishes.

Also fix DecryptingDemuxerStream that when Reset() is called during a pending
config-change read, we should return kConfigChanged back to the caller rather
than return a kAborted.

BUG=304260, 303835
TEST=Added unittests to cover these cases.
R=scherkus@chromium.org

Review URL: https://codereview.chromium.org/26588003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227879 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227879