IntersectionObserver: Fixes |unobserve| to take effect immediately.

Applies the same fix as https://crrev.com/c/924104 to
IntersectionObserver, too.

Unfortunately, I cannot reproduce the crash for IntersectionObserver,
and I'm not sure if this fix is actually effective or not.  However,
seeing the same fix worked (at least partially) for ResizeObserver,
I'd like to apply the same fix to IntersectionObserver, too.

Bug: 815058
Change-Id: I0e89cd56ab4915457178b38623daadc0be20c927
Reviewed-on: https://chromium-review.googlesource.com/934483
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539094}

third_party/WebKit/Source/core/intersection_observer/IntersectionObserver.cpp

@@ -273,10 +273,19 @@ void IntersectionObserver::unobserve(Element* target,
   if (!target || !target->IntersectionObserverData())
     return;
 
-  if (IntersectionObservation* observation =
-          target->IntersectionObserverData()->GetObservationFor(*this)) {
-    observation->Disconnect();
-    observations_.erase(observation);
+  IntersectionObservation* observation =
+      target->IntersectionObserverData()->GetObservationFor(*this);
+  if (!observation)
+    return;
+
+  observation->Disconnect();
+  observations_.erase(observation);
+
+  for (size_t i = 0; i < entries_.size(); ++i) {
+    if (entries_[i]->target() == target) {
+      entries_.EraseAt(i);
+      --i;
+    }
   }
 }