Forbid ImageDownloaderImpl from accessing RenderThread instance after message loop shutdown.

content/renderer/image_downloader/image_downloader_impl.cc

Index: content/renderer/image_downloader/image_downloader_impl.cc
diff --git a/content/renderer/image_downloader/image_downloader_impl.cc b/content/renderer/image_downloader/image_downloader_impl.cc
index ac4995a2fec9d3343a410706ee1635a4c103f67d..31e9c51fd271c588ec34db957b8f2e4f78d585ea 100644
--- a/content/renderer/image_downloader/image_downloader_impl.cc
+++ b/content/renderer/image_downloader/image_downloader_impl.cc
@@ -130,7 +130,9 @@ ImageDownloaderImpl::ImageDownloaderImpl(
 }
 
 ImageDownloaderImpl::~ImageDownloaderImpl() {
-  RenderThread::Get()->RemoveObserver(this);
+  RenderThread* thread = RenderThread::Get();
+  if (thread)

[yzshen1, 2016/08/26 19:20:31]

Please comment on why this null check is necessary

[leonhsl(Using Gerrit), 2016/08/29 02:17:30]

Done and thanks.

+    thread->RemoveObserver(this);
 }
 
 // static