[inspector] speculative avoid using debugger context as current in blink

Now we run nested message loop with debugger context as current one.

BUG=chromium:676749
R=yangguo@chromium.org,dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2604553002
Cr-Commit-Position: refs/heads/master@{#41937}

src/inspector/v8-debugger.cc

@@ -488,6 +488,10 @@ void V8Debugger::handleProgramBreak(v8::Local<v8::Context> pausedContext,
     m_runningNestedMessageLoop = true;
     int groupId = m_inspector->contextGroupId(pausedContext);
     DCHECK(groupId);
+    v8::Context::Scope scope(pausedContext);
+    v8::Local<v8::Context> context = m_isolate->GetCurrentContext();
+    CHECK(!context.IsEmpty() &&
+          context != v8::Debug::GetDebugContext(m_isolate));
     m_inspector->client()->runMessageLoopOnPause(groupId);
     // The agent may have been removed in the nested loop.
     agent = m_inspector->enabledDebuggerAgentForGroup(