Fix assert with distiller code caused by r419635.

components/dom_distiller/content/browser/dom_distiller_viewer_source.cc

Index: components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
diff --git a/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc b/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
index 0669e214b03631cfa70e60ee9c342f868a7a7a44..0058d6881072d43b55129c03dca9a7cd4c984690 100644
--- a/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
+++ b/components/dom_distiller/content/browser/dom_distiller_viewer_source.cc
@@ -132,6 +132,9 @@ void DomDistillerViewerSource::RequestViewerHandle::SendJavaScript(
 
 void DomDistillerViewerSource::RequestViewerHandle::DidFinishNavigation(
     content::NavigationHandle* navigation_handle) {
+  if (!navigation_handle->IsInMainFrame() || !navigation_handle->HasCommitted())
+    return;
+
   const GURL& navigation = navigation_handle->GetURL();
   bool expected_main_view_request =
       navigation.SchemeIs(expected_scheme_.c_str()) &&