Prepare WebCore.gyp for ffmpeg source transition.

Prepare WebCore.gyp for ffmpeg source transition.
https://bugs.webkit.org/show_bug.cgi?id=77254

We're migrating our ffmpeg repo from a set of patches living on top of
a tarball to an actual git fork of upstream.  The paths have changed
slightly.

In order to not break the current build, we'll keep both paths around
until the transition is complete.  Afterward the 'patched-ffmpeg' path
will be removed.

Patch by Dale Curtis <dalecurtis@chromium.org> on 2012-01-29
Reviewed by Tony Chang.

No new tests. GYP change, if it doesn't work, nothing will compile.

* WebCore.gyp/WebCore.gyp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106209 268f45cc-cd09-0410-ab3c-d52691b4dbfc