Null-check |document_| in IncrementLoadEventDelayCount

After https://chromium-review.googlesource.com/720640, |document_|
is WeakPersistent and thus it should be null-checked before use.

Bug: 773571, 775090
Change-Id: I914b57071fa07288997162d8645bab4ded440032
Reviewed-on: https://chromium-review.googlesource.com/721681
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Yuzu Saijo <yuzus@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509567}

third_party/WebKit/Source/core/dom/IncrementLoadEventDelayCount.cpp

@@ -26,7 +26,8 @@ IncrementLoadEventDelayCount::~IncrementLoadEventDelayCount() {
 }
 
 void IncrementLoadEventDelayCount::ClearAndCheckLoadEvent() {
-  document_->DecrementLoadEventDelayCountAndCheckLoadEvent();
+  if (document_)
+    document_->DecrementLoadEventDelayCountAndCheckLoadEvent();
   document_ = nullptr;
 }