Implement HTMLMediaElement::srcObject.

Source/core/html/HTMLMediaElement.idl

Index: Source/core/html/HTMLMediaElement.idl
diff --git a/Source/core/html/HTMLMediaElement.idl b/Source/core/html/HTMLMediaElement.idl
index 7990f6b726eb6a934d234c1fc82594960b594d34..d98dbbd8399ff05d3d150c6e114d01258ad45e42 100644
--- a/Source/core/html/HTMLMediaElement.idl
+++ b/Source/core/html/HTMLMediaElement.idl
@@ -22,6 +22,10 @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+ 
+// FIXME: should use typedef instead, but
+// requires better union type support: http://crbug.com/240176
+typedef (MediaStream or MediaSource or Blob) MediaProvider;
 
 [
     ActiveDOMObject,
@@ -35,6 +39,8 @@
 
     // network state
     [Reflect, URL] attribute DOMString src;
+    // FIXME: should be attribute MediaProvider? srcObject;　ｈttp://crbug.com/240176
+    [Custom] attribute object? srcObject;    
     [URL] readonly attribute DOMString currentSrc;
     [Reflect, ReflectOnly=("anonymous","use-credentials"), ReflectEmpty="anonymous", ReflectInvalid="anonymous"] attribute DOMString? crossOrigin;