Swap the main frame in --site-per-process mode too.

Calling swap() is required to set up a lot of required state in Blink
for the WebRemoteFrame. Without this, bad things happen.

BUG=475003

Review URL: https://codereview.chromium.org/1106673002

Cr-Commit-Position: refs/heads/master@{#329012}

content/browser/site_per_process_browsertest.cc

@@ -2395,4 +2395,42 @@ IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest, RFPHDestruction) {
       DepictFrameTree(root));
 }
 
+// Test that opening a cross-site window with child frames doesn't crash either
+// renderer in --site-per-process. This was previously broken because frame
+// swaps weren't being performed on main frames, so it would crash when trying
+// to mirror the child frames. https://crbug.com/475003
+IN_PROC_BROWSER_TEST_F(SitePerProcessBrowserTest,
+                       OpenCrossSiteWindowWithFrames) {
+  ASSERT_EQ(1u, Shell::windows().size());
+  Shell* main_window = Shell::windows()[0];
+
+  // Navigate the main window.
+  GURL main_url(embedded_test_server()->GetURL("/site_per_process_main.html"));
+  NavigateToURL(main_window, main_url);
+  EXPECT_EQ(main_window->web_contents()->GetLastCommittedURL(), main_url);
+
+  // Load a cross-site page into a new window.
+  GURL cross_url =
+      embedded_test_server()->GetURL("foo.com", "/site_per_process_main.html");
+  std::string script = "window.open('" + cross_url.spec() + "')";
+  EXPECT_TRUE(ExecuteScript(main_window->web_contents(), script));
+  ASSERT_EQ(2u, Shell::windows().size());
+  Shell* cross_window = Shell::windows()[1];
+  WaitForLoadStop(cross_window->web_contents());
+  EXPECT_EQ(cross_window->web_contents()->GetLastCommittedURL(), cross_url);
+
+  // Make sure the main window is still live. To create a synchronization point
+  // and ensure that the renderer has processed all the messages for updating
+  // the frame tree, execute a simple bit of JS first.
+  EXPECT_TRUE(ExecuteScript(main_window->web_contents(), "true"));
+  EXPECT_TRUE(static_cast<WebContentsImpl*>(main_window->web_contents())
+                  ->GetMainFrame()
+                  ->IsRenderFrameLive());
+  // And check the cross-site window too.
+  EXPECT_TRUE(ExecuteScript(cross_window->web_contents(), "true"));
+  EXPECT_TRUE(static_cast<WebContentsImpl*>(cross_window->web_contents())
+                  ->GetMainFrame()
+                  ->IsRenderFrameLive());
+}
+
 }  // namespace content

content/renderer/render_frame_impl.cc

@@ -1227,7 +1227,7 @@ void RenderFrameImpl::OnSwapOut(
   // Now that all of the cleanup is complete and the browser side is notified,
   // start using the RenderFrameProxy, if one is created.
   if (proxy) {
-    if (!is_main_frame) {
+    if (!is_main_frame || is_site_per_process) {
       frame_->swap(proxy->web_frame());
 
       if (is_loading)