Fixed crash with Multisample observer

BUG=333040

Review URL: https://codereview.chromium.org/214413010

git-svn-id: svn://svn.chromium.org/blink/trunk@170253 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/page/Page.cpp

@@ -220,6 +220,7 @@ void Page::setMainFrame(PassRefPtr<LocalFrame> mainFrame)
 
 void Page::documentDetached(Document* document)
 {
+    m_multisamplingChangedObservers.clear();
     m_pointerLockController->documentDetached(document);
     m_contextMenuController->documentDetached(document);
     if (m_validationMessageClient)