[Web Animations] DocumentTimeline shouldn't suspend itself if hiddenPageCSSAnimationSuspensionEnabled is disabled

https://bugs.webkit.org/show_bug.cgi?id=207014
<rdar://problem/58815952>

Reviewed by Antti Koivisto.

We suspend a timeline upon consutrction if we know that the page is not visible because, unlike CSSAnimationController, the DocumentTimeline is not guaranteed
to be created by the time the Page sets the initial visibility state. This is because DocumentTimeline is created as needed when there are CSS Animations or CSS
Transitions created for the page, or if the content uses any of the Web Animations APIs.

However, the Page::setIsVisibleInternal() function that would call DocumentTimeline::resumeAnimations() at a later time checks whether the hiddenPageCSSAnimationSuspensionEnabled
setting is enabled. So we must respect that setting also when suspending animations in the first place or we risk ending up in a state where we suspend animations
because the page is not visible upon timeline creation, but never resuming animations later due to the hiddenPageCSSAnimationSuspensionEnabled setting being false.

* animation/DocumentTimeline.cpp:
(WebCore::DocumentTimeline::DocumentTimeline):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@255489 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Author: [email protected]

Source/WebCore/ChangeLog

@@ -1,3 +1,22 @@
+2020-01-30  Antoine Quint  <[email protected]>
+
+        [Web Animations] DocumentTimeline shouldn't suspend itself if hiddenPageCSSAnimationSuspensionEnabled is disabled
+        https://bugs.webkit.org/show_bug.cgi?id=207014
+        <rdar://problem/58815952>
+
+        Reviewed by Antti Koivisto.
+
+        We suspend a timeline upon consutrction if we know that the page is not visible because, unlike CSSAnimationController, the DocumentTimeline is not guaranteed
+        to be created by the time the Page sets the initial visibility state. This is because DocumentTimeline is created as needed when there are CSS Animations or CSS
+        Transitions created for the page, or if the content uses any of the Web Animations APIs.
+
+        However, the Page::setIsVisibleInternal() function that would call DocumentTimeline::resumeAnimations() at a later time checks whether the hiddenPageCSSAnimationSuspensionEnabled
+        setting is enabled. So we must respect that setting also when suspending animations in the first place or we risk ending up in a state where we suspend animations
+        because the page is not visible upon timeline creation, but never resuming animations later due to the hiddenPageCSSAnimationSuspensionEnabled setting being false.
+
+        * animation/DocumentTimeline.cpp:
+        (WebCore::DocumentTimeline::DocumentTimeline):
+
 2020-01-31  Jiewen Tan  <[email protected]>
 
         REGRESSION: [iOS release] http/tests/security/window-named-proto.html is a flaky timing out

Source/WebCore/animation/DocumentTimeline.cpp

@@ -43,6 +43,7 @@
 #include "RenderElement.h"
 #include "RenderLayer.h"
 #include "RenderLayerBacking.h"
+#include "Settings.h"
 #include <JavaScriptCore/VM.h>
 
 namespace WebCore {
@@ -63,10 +64,13 @@ DocumentTimeline::DocumentTimeline(Document& document, Seconds originTime)
     , m_document(&document)
     , m_originTime(originTime)
 {
-    if (m_document)
+    if (m_document) {
         m_document->addTimeline(*this);
-    if (m_document && m_document->page() && !m_document->page()->isVisible())
-        suspendAnimations();
+        if (auto* page = m_document->page()) {
+            if (page->settings().hiddenPageCSSAnimationSuspensionEnabled() && !page->isVisible())
+                suspendAnimations();
+        }
+    }
 }
 
 DocumentTimeline::~DocumentTimeline()