[DevTools] Clear DatabaseClient's pointer to InspectorDatabaseAgent on detach.

This should not matter in regular case, as the Page, DatabaseClient and
InspectorDatabaseAgent all have the same lifetime.
But it looks like we can have DevTools initialized twice for the page in OOPIF case.

BUG=526162

Review URL: https://codereview.chromium.org/1323443004

git-svn-id: svn://svn.chromium.org/blink/trunk@201569 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/modules/webdatabase/DatabaseClient.cpp

@@ -70,7 +70,7 @@ void DatabaseClient::didOpenDatabase(Database* database, const String& domain, c
 
 void DatabaseClient::setInspectorAgent(InspectorDatabaseAgent* agent)
 {
-    ASSERT(!m_inspectorAgent);
+    // TODO(dgozman): we should not set agent twice, but it's happening in OOPIF case.
     m_inspectorAgent = agent;
 }