Add MediaSession runtime flag along with stub MediaSession object

Source/modules/mediasession/MediaSession.idl

Index: Source/modules/mediasession/MediaSession.idl
diff --git a/Source/modules/mediasession/MediaSession.idl b/Source/modules/mediasession/MediaSession.idl
new file mode 100644
index 0000000000000000000000000000000000000000..111bddcedf50cad92656e1490a3f196f5c887afe
--- /dev/null
+++ b/Source/modules/mediasession/MediaSession.idl
@@ -0,0 +1,8 @@
+[
+    Constructor(),
+    RuntimeEnabled=MediaSession,
+]
+interface MediaSession {

[philipj_slow, 2015/06/26 11:56:59]

The usual thing is to put the ] on the same line as "interface", although
certainly not in specs, just in Blink :/

[davve, 2015/06/26 12:50:50]

Done.

+    void activate();
+    void deactivate();
+};