Add minimal implementation of unprefixed MediaSource API that has feature parity with prefixed API

Source/modules/modules.gypi

Index: Source/modules/modules.gypi
diff --git a/Source/modules/modules.gypi b/Source/modules/modules.gypi
index 551dd63a2b4714bb4ba01284ceb065e5728f79e7..1fe96c95edfa229a76efed21a8a883869f18fa8a 100644
--- a/Source/modules/modules.gypi
+++ b/Source/modules/modules.gypi
@@ -60,6 +60,9 @@
       'indexeddb/IDBVersionChangeEvent.idl',
       'indexeddb/WorkerContextIndexedDatabase.idl',
       'mediasource/DOMWindowMediaSource.idl',
+      'mediasource/MediaSource.idl',
+      'mediasource/SourceBuffer.idl',
+      'mediasource/SourceBufferList.idl',
       'mediasource/WebKitMediaSource.idl',
       'mediasource/WebKitSourceBuffer.idl',
       'mediasource/WebKitSourceBufferList.idl',
@@ -322,10 +325,16 @@
       'indexeddb/WorkerContextIndexedDatabase.h',
       'indexeddb/chromium/IDBFactoryBackendInterfaceChromium.cpp',
       'indexeddb/chromium/IDBFactoryBackendInterfaceChromium.h',
+      'mediasource/MediaSource.cpp',
+      'mediasource/MediaSource.h',
       'mediasource/MediaSourceBase.cpp',
       'mediasource/MediaSourceBase.h',
       'mediasource/MediaSourceRegistry.cpp',
       'mediasource/MediaSourceRegistry.h',
+      'mediasource/SourceBuffer.cpp',
+      'mediasource/SourceBuffer.h',
+      'mediasource/SourceBufferList.cpp',
+      'mediasource/SourceBufferList.h',
       'mediasource/WebKitMediaSource.cpp',
       'mediasource/WebKitMediaSource.h',
       'mediasource/WebKitSourceBuffer.cpp',