Fix renderer process host backgrounding test on Windows.

chrome/browser/renderer_host/render_process_host_chrome_browsertest.cc

Index: chrome/browser/renderer_host/render_process_host_chrome_browsertest.cc
diff --git a/chrome/browser/renderer_host/render_process_host_chrome_browsertest.cc b/chrome/browser/renderer_host/render_process_host_chrome_browsertest.cc
index b2143f955f0ed97186be96aa7e197e33212a2916..6f1aa74ce434db0ef736523c665f95d0587abe76 100644
--- a/chrome/browser/renderer_host/render_process_host_chrome_browsertest.cc
+++ b/chrome/browser/renderer_host/render_process_host_chrome_browsertest.cc
@@ -21,6 +21,7 @@
 #include "content/public/browser/render_widget_host_iterator.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/browser/web_contents_observer.h"
+#include "content/public/test/browser_test_utils.h"
 
 using content::RenderViewHost;
 using content::RenderWidgetHost;
@@ -72,6 +73,7 @@ class ChromeRenderProcessHostTest : public InProcessBrowserTest {
     CHECK(wc->GetURL() == page);
 
     WaitForLauncherThread();
+    WaitForMessageProcessing(wc);
     return wc->GetRenderProcessHost()->GetHandle();
   }
 
@@ -87,6 +89,7 @@ class ChromeRenderProcessHostTest : public InProcessBrowserTest {
     CHECK(wc->GetVisibleURL() == page);
 
     WaitForLauncherThread();
+    WaitForMessageProcessing(wc);
     return wc->GetRenderProcessHost()->GetHandle();
   }
 
@@ -98,6 +101,15 @@ class ChromeRenderProcessHostTest : public InProcessBrowserTest {
     base::MessageLoop::current()->Run();
   }
 
+  // Implicitly waits for the given WebContents to process outstanding IPC
+  // messages by running some JavaScript and waiting for the result.

[jam, 2014/06/04 01:06:05]

this comment is a bit misleading; afaik you're waiting for the renderer to
process an IPC, and not a WebContents (browser process) right? if so, adjust the
comment to say that this ensures that the renderer processed the IPC to set
itself to be backgrounded

[DaleCurtis, 2014/06/04 01:10:18]

Done.

+  void WaitForMessageProcessing(WebContents* wc) {
+    bool result = false;
+    ASSERT_TRUE(content::ExecuteScriptAndExtractBool(
+        wc, "window.domAutomationController.send(true);", &result));
+    ASSERT_TRUE(result);
+  }
+
   // When we hit the max number of renderers, verify that the way we do process
   // sharing behaves correctly.  In particular, this test is verifying that even
   // when we hit the max process limit, that renderers of each type will wind up
@@ -252,9 +264,7 @@ IN_PROC_BROWSER_TEST_F(ChromeRenderProcessHostTest, ProcessPerTab) {
 
 // We don't change process priorities on Mac or Posix because the user lacks the
 // permission to raise a process' priority even after lowering it.
-// TODO(dalecurtis): Reenable this on Windows after figuring out how to reliably
-// wait for the renderer process to process IPC messages.
-#if defined(OS_LINUX)
+#if defined(OS_WIN) || defined(OS_LINUX)
 IN_PROC_BROWSER_TEST_F(ChromeRenderProcessHostTest, Backgrounding) {
   if (!base::Process::CanBackgroundProcesses()) {
     LOG(ERROR) << "Can't background processes";