Improve forward seeking in SMILTimeContainer

When seeking forward we don't need to reset all the timed elements, but
can just move from the latest update time to the new presentation time
directly.

Bug: 998526
Change-Id: I1866b59b4e02f9de1a26c6a5844c4d8893dd68b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856161Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#705393}

third_party/blink/renderer/core/svg/animation/smil_time_container.cc

@@ -275,16 +275,21 @@ void SMILTimeContainer::SetElapsed(SMILTime elapsed) {
   if (!IsPaused())
     SynchronizeToDocumentTimeline();
 
-  {
+  // If we are rewinding the timeline, we need to start from 0 and then move
+  // forward to the new presentation time. If we're moving forward we can just
+  // perform the update in the normal fashion.
+  if (elapsed < latest_update_time_) {
     base::AutoReset<bool> updating_intervals_scope(&is_updating_intervals_,
                                                    true);
     ScheduledAnimationsMutationsForbidden scope(this);
     for (auto& sandwich : scheduled_animations_.Values())
       sandwich->Reset();
-  }
 
+    latest_update_time_ = SMILTime();
+  }
+  // We need to set this to make sure we update the active state of the timed
+  // elements.
   intervals_dirty_ = true;
-  latest_update_time_ = SMILTime();
   UpdateAnimationsAndScheduleFrameIfNeeded(elapsed);
 }