Web Animations: Timeline should not advance during task execution

Source/web/WebKit.cpp

Index: Source/web/WebKit.cpp
diff --git a/Source/web/WebKit.cpp b/Source/web/WebKit.cpp
index aa722a53f5ba662b3636d54f163e60608a5afc34..fb5c54457ec4163422a07915cd8668da6a32788f 100644
--- a/Source/web/WebKit.cpp
+++ b/Source/web/WebKit.cpp
@@ -35,6 +35,7 @@
 #include "bindings/v8/V8Binding.h"
 #include "bindings/v8/V8Initializer.h"
 #include "core/Init.h"
+#include "core/animation/AnimationClock.h"
 #include "core/dom/Microtask.h"
 #include "core/frame/Settings.h"
 #include "core/page/Page.h"
@@ -66,7 +67,10 @@ namespace {
 
 class EndOfTaskRunner : public WebThread::TaskObserver {

[dstockwell, 2014/04/30 12:07:22]

We probably want to rename this, or create a separate observer. Suggestions?

 public:
-    virtual void willProcessTask() { }
+    virtual void willProcessTask()
+    {
+        WebCore::AnimationClock::notifyTaskStartTime(monotonicallyIncreasingTime());

[esprehn, 2014/04/30 22:43:50]

How expensive is monotonicallyIncreasingTime? Can we freeze the animation clock
at frame boundaries and unfreeze always at end of microtask only if it's already
frozen?

[dstockwell, 2014/04/30 22:59:51]

I will investigate.
I don't think this helps. For any task which is not a frame task we still need
an up to date time value. A significant amount of time might pass before the
next frame.

+    }

[abarth-chromium, 2014/04/30 22:39:16]

Doesn't this make the entire engine slower?  We execute many, many tasks...

[dstockwell, 2014/05/01 00:22:21]

I'm going to try a new approach which should reduce this to an increment before
the task, or require a micro task.

     virtual void didProcessTask()
     {
         WebCore::Microtask::performCheckpoint();