Add srcObject attribute of type MediaStream to HTMLMediaElement.

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 309 matching lines @@
       'exported/WebFederatedCredential.cpp',
       'exported/WebFileSystemCallbacks.cpp',
       'exported/WebFont.cpp',
       'exported/WebFontDescription.cpp',
       'exported/WebHTTPBody.cpp',
       'exported/WebHTTPLoadInfo.cpp',
       'exported/WebImageGenerator.cpp',
       'exported/WebImage.cpp',
       'exported/WebMediaConstraints.cpp',
       'exported/WebMediaDeviceInfo.cpp',
+      'exported/WebMediaElementSource.cpp',
       'exported/WebMediaPlayerClient.cpp',
       'exported/WebMediaPlayerEncryptedMediaClient.cpp',
       'exported/WebMediaRecorderHandler.cpp',
       'exported/WebMediaStream.cpp',
       'exported/WebMediaStreamSource.cpp',
       'exported/WebMediaStreamTrack.cpp',
       'exported/WebMediaStreamTrackSourcesRequest.cpp',
       'exported/WebMemoryAllocatorDump.cpp',
       'exported/WebMemoryDumpProvider.cpp',
       'exported/WebMessagePortChannelClient.cpp',
@@ skipping 946 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }