Start autoplay muted videos with autoplay attribute when they are visible.

third_party/WebKit/Source/core/core.gypi

 {
     'includes': [
         'core_generated.gypi',
     ],
     'variables': {
         # Files for which bindings (.cpp and .h files) will be generated
         'core_idl_files': [
             'animation/KeyframeEffect.idl',
             'animation/EffectModel.idl',
             'animation/AnimationEffectReadOnly.idl',
@@ skipping 2466 matching lines @@
             'dom/Element.h',
             'dom/ElementData.cpp',
             'dom/ElementData.h',
             'dom/ElementDataCache.cpp',
             'dom/ElementDataCache.h',
             'dom/ElementFullscreen.cpp',
             'dom/ElementFullscreen.h',
             'dom/ElementRareData.cpp',
             'dom/ElementRareData.h',
             'dom/ElementTraversal.h',
+            'dom/ElementVisibilityObserver.cpp',
+            'dom/ElementVisibilityObserver.h',
             'dom/EmptyNodeList.cpp',
             'dom/EmptyNodeList.h',
             'dom/ExecutionContext.cpp',
             'dom/ExecutionContext.h',
             'dom/ExecutionContextTask.h',
             'dom/FirstLetterPseudoElement.cpp',
             'dom/FirstLetterPseudoElement.h',
             'dom/FlexibleArrayBufferView.h',
             'dom/FrameRequestCallback.h',
             'dom/FrameRequestCallbackCollection.cpp',
@@ skipping 1680 matching lines @@
             'testing/PrivateScriptTestTest.cpp',
             'timing/MemoryInfoTest.cpp',
             'timing/PerformanceBaseTest.cpp',
             'timing/PerformanceObserverTest.cpp',
             'workers/WorkerThreadTest.cpp',
             'workers/WorkerThreadTestHelper.h',
             'xml/parser/SharedBufferReaderTest.cpp',
         ],
     }
 }