[Mac][EME] Query whether decoder support exists before creating a new MediaKeys object.

[Mac][EME] Query whether decoder support exists before creating a new MediaKeys object.
https://bugs.webkit.org/show_bug.cgi?id=130459

Reviewed by Eric Carlson.

Source/WebCore:

* WebCore.exp.in: Export the new WCSI method.
* platform/graphics/avfoundation/objc/MediaPlayerPrivateMediaSourceAVFObjC.mm:
(WebCore::MediaPlayerPrivateMediaSourceAVFObjC::supportsKeySystem): Bail early if we have no decoder support.
* platform/mac/WebCoreSystemInterface.h: Provide a WCSI wrapper for WKSI.
* platform/mac/WebCoreSystemInterface.mm: Ditto.

Source/WebKit/mac:

* WebCoreSupport/WebSystemInterface.mm:
(InitWebCoreSystemInterface): Initialize the new WCSI method.

Source/WebKit2:

* WebProcess/WebCoreSupport/mac/WebSystemInterface.mm:
(InitWebCoreSystemInterface): Initialize the new WCSI method.

WebKitLibraries:

* WebKitSystemInterface.h:
* libWebKitSystemInterfaceMavericks.a:
* libWebKitSystemInterfaceMountainLion.a:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165917 268f45cc-cd09-0410-ab3c-d52691b4dbfc