Web Animations: Avoid leak in AnimationPlayer

AnimationPlayer should stop indicating activity when active dom object is
stopped.

BUG=379032

Review URL: https://codereview.chromium.org/301363002

git-svn-id: svn://svn.chromium.org/blink/trunk@175143 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/LayoutTests/LeakExpectations

@@ -38,8 +38,6 @@ crbug.com/327574 permissionclient/image-permissions.html [ Leak ]
 crbug.com/327574 http/tests/permissionclient/image-permissions.html [ Leak ]
 crbug.com/327574 fast/loader/data-images-with-images-disabled.html [ Leak ]
 
-crbug.com/379032 web-animations-api/player-finish-sample-only.html [ Leak ]
-
 # -----------------------------------------------------------------
 # Untriaged but known leaks which may be false positives.
 # -----------------------------------------------------------------

third_party/WebKit/Source/core/animation/AnimationPlayer.cpp

@@ -298,6 +298,7 @@ bool AnimationPlayer::hasPendingActivity() const
 
 void AnimationPlayer::stop()
 {
+    m_finished = true;
     m_pendingFinishedEvent = nullptr;
 }