Sspeculative fix for a crash in DevToolsTargetRegistry::ContentObserver

Bug: 838777
Change-Id: I8a1a049ade70cadfebabc8115471732f15432ac1
Reviewed-on: https://chromium-review.googlesource.com/1058372
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558736}

content/browser/devtools/devtools_target_registry.cc

@@ -149,10 +149,12 @@ class DevToolsTargetRegistry::ContentsObserver : public ObserverBase,
   void WebContentsDestroyed() override {
     NOTREACHED() << "DevToolsTarget Registry clients should be destroyed "
                     "before WebContents";
+    registry_->UnregisterWebContents(web_contents());
   }
 
   ~ContentsObserver() override {
-    registry_->UnregisterWebContents(web_contents());
+    if (web_contents())
+      registry_->UnregisterWebContents(web_contents());
   }
 
   DevToolsTargetRegistry* registry_;