Implement HTML video and audio element lazy-loading via the loading attribute

[credod]

59d3992

Implement HTML video and audio element lazy-loading via the loading attribute
https://bugs.webkit.org/show_bug.cgi?id=303995

Reviewed by NOBODY (OOPS!).

Implement proposed loading attribute for video and audio elements.
This patch implements a proposed loading attribute for video and audio elements, enabling lazy loading of media sources and video poster images, and deferring autoplay.

This change is proposed to be added to the HTML spec in the following PR:
whatwg/html#11980

This patch is passing video and audio WPT tests from the following PR:
web-platform-tests/wpt#57051

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:
* Source/WebCore/Sources.txt:
* Source/WebCore/WebCore.xcodeproj/project.pbxproj:
* Source/WebCore/dom/Document.cpp:
(WebCore::Document::lazyLoadMediaObserver):
* Source/WebCore/dom/Document.h:
* Source/WebCore/html/HTMLMediaElement.cpp:
(WebCore::HTMLMediaElement::didMoveToNewDocument):
(WebCore::HTMLMediaElement::attributeChanged):
(WebCore::HTMLMediaElement::didFinishInsertingNode):
(WebCore::HTMLMediaElement::removedFromAncestor):
(WebCore::HTMLMediaElement::hasLazyLoadableAttributeValue):
(WebCore::HTMLMediaElement::isLazyLoadable const):
(WebCore::HTMLMediaElement::loadDeferredMedia):
(WebCore::HTMLMediaElement::resumeLazyLoadingIfNeeded):
(WebCore::HTMLMediaElement::loading const):
(WebCore::HTMLMediaElement::setLoading):
(WebCore::HTMLMediaElement::play):
(WebCore::HTMLMediaElement::sourceWasAdded):
(WebCore::HTMLMediaElement::setShouldDelayLoadEvent):
* Source/WebCore/html/HTMLMediaElement.h:
* Source/WebCore/html/HTMLMediaElement.idl:
* Source/WebCore/html/HTMLVideoElement.cpp:
(WebCore::HTMLVideoElement::rendererIsNeeded):
(WebCore::HTMLVideoElement::supportsFullscreen const):
(WebCore::HTMLVideoElement::hasAvailableVideoFrame const):
(WebCore::HTMLVideoElement::webkitEnterFullscreen):
(WebCore::HTMLVideoElement::loadDeferredMedia):
* Source/WebCore/html/HTMLVideoElement.h:
* Source/WebCore/html/LazyLoadMediaObserver.cpp: Added.
(WebCore::LazyLoadMediaObserver::observe):
(WebCore::LazyLoadMediaObserver::unobserve):
(WebCore::LazyLoadMediaObserver::intersectionObserver):
(WebCore::LazyLoadMediaObserver::isObserved const):
* Source/WebCore/html/LazyLoadMediaObserver.h: Added.
* Source/WebCore/html/parser/HTMLParserOptions.cpp:
* Source/WebCore/html/parser/HTMLPreloadScanner.cpp:
(WebCore::TokenPreloadScanner::StartTagScanner::processAttribute):
* Source/WebCore/html/shadow/DataListButtonElement.cpp:
* Source/WebCore/html/shadow/SpinButtonElement.cpp:
* Source/WebCore/loader/ImageLoader.cpp:
(WebCore::ImageLoader::updateFromElement):
(WebCore::ImageLoader::didUpdateCachedImage):
(WebCore::ImageLoader::notifyFinished):
(WebCore::ImageLoader::updatedHasPendingEvent):
(WebCore::ImageLoader::decode):

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