[inspector] fixed injected-script-host crash

It's possible to get undefined as a result of Object.getOwnPropertyDescriptor. We should be ready for this.

BUG=chromium:707670
R=alph@chromium.org

Review-Url: https://codereview.chromium.org/2801763002
Cr-Commit-Position: refs/heads/master@{#44532}

src/inspector/injected-script-source.js

@@ -403,7 +403,9 @@ InjectedScript.prototype = {
                 var descriptor;
                 try {
                     descriptor = InjectedScriptHost.getOwnPropertyDescriptor(o, property);
-                    InjectedScriptHost.nullifyPrototype(descriptor);
+                    if (descriptor) {
+                        InjectedScriptHost.nullifyPrototype(descriptor);
+                    }
                     var isAccessorProperty = descriptor && ("get" in descriptor || "set" in descriptor);
                     if (accessorPropertiesOnly && !isAccessorProperty)
                         continue;

src/inspector/v8-injected-script-host.cc

@@ -105,7 +105,9 @@ v8::Local<v8::Object> V8InjectedScriptHost::create(
 
 void V8InjectedScriptHost::nullifyPrototypeCallback(
     const v8::FunctionCallbackInfo<v8::Value>& info) {
-  CHECK(info.Length() == 1 && info[0]->IsObject());
+  CHECK(info.Length() == 1);
+  DCHECK(info[0]->IsObject());
+  if (!info[0]->IsObject()) return;
   v8::Isolate* isolate = info.GetIsolate();
   info[0]
       .As<v8::Object>()