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..5cb89edfb2a59ae55b4cdc8194bdfcfd6c9626ac 100644
--- a/content/renderer/image_downloader/image_downloader_impl.cc
+++ b/content/renderer/image_downloader/image_downloader_impl.cc
@@ -130,7 +130,12 @@ ImageDownloaderImpl::ImageDownloaderImpl(
 }
 
 ImageDownloaderImpl::~ImageDownloaderImpl() {
-  RenderThread::Get()->RemoveObserver(this);
+  RenderThread* thread = RenderThread::Get();
+  // As ImageDownloaderImpl is a strong binding with message pipe, the
+  // destructor may run after message loop shutdown, so we need to check whether
+  // RenderThread is null.
+  if (thread)
+    thread->RemoveObserver(this);
 }
 
 // static