Also ship ffmpegsumo.dll in the distribution.

atom.gyp

@@ -275,6 +275,7 @@
               'destination': '<(PRODUCT_DIR)',
               'files': [
                 '<(libchromiumcontent_library_dir)/chromiumcontent.dll',
+                '<(libchromiumcontent_library_dir)/ffmpegsumo.dll',
                 '<(libchromiumcontent_library_dir)/icudt.dll',
                 '<(libchromiumcontent_library_dir)/libGLESv2.dll',
                 '<(libchromiumcontent_resources_dir)/content_shell.pak',

script/create-dist.py

@@ -34,6 +34,7 @@ TARGET_BINARIES = {
     'atom.exe',
     'chromiumcontent.dll',
     'content_shell.pak',
+    'ffmpegsumo.dll',
     'icudt.dll',
     'libGLESv2.dll',
   ],