clean up replace contents()->GetMainFrame() to main_test_rfh()

content/browser/web_contents/web_contents_impl_unittest.cc

Index: content/browser/web_contents/web_contents_impl_unittest.cc
diff --git a/content/browser/web_contents/web_contents_impl_unittest.cc b/content/browser/web_contents/web_contents_impl_unittest.cc
index 9228766d4f31d81c16ca2fd49c6184cd5c808cb9..f027f0feeb9cc716835ee6959fe74fbbf8699b85 100644
--- a/content/browser/web_contents/web_contents_impl_unittest.cc
+++ b/content/browser/web_contents/web_contents_impl_unittest.cc
@@ -2934,7 +2934,7 @@ TEST_F(WebContentsImplTest, ActiveContentsCountNavigate) {
                                     std::string());
   int entry_id = contents->GetController().GetPendingEntry()->GetUniqueID();
   if (IsBrowserSideNavigationEnabled())
-    contents->GetMainFrame()->PrepareForCommit();
+    main_test_rfh()->PrepareForCommit();

[ncarter (slow), 2016/09/27 20:47:56]

Oh yeah, this isn't safe at all.

   EXPECT_TRUE(contents->CrossProcessNavigationPending());
   EXPECT_EQ(1u, instance->GetRelatedActiveContentsCount());
   contents->GetPendingMainFrame()->SendNavigate(1, entry_id, true, kUrl);
@@ -2971,7 +2971,7 @@ TEST_F(WebContentsImplTest, ActiveContentsCountChangeBrowsingInstance) {
                                     ui::PAGE_TRANSITION_TYPED,
                                     std::string());
   int entry_id = contents->GetController().GetPendingEntry()->GetUniqueID();
-  contents->GetMainFrame()->PrepareForCommit();
+  main_test_rfh()->PrepareForCommit();
   EXPECT_TRUE(contents->CrossProcessNavigationPending());
   scoped_refptr<SiteInstance> instance_webui(
       contents->GetPendingMainFrame()->GetSiteInstance());