Debug flaky gpu pixel tests by printing event trace on failure.

chrome/test/gpu/gpu_pixel_browsertest.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/command_line.h"
 #include "base/file_path.h"
 #include "base/file_util.h"
 #include "base/path_service.h"
 #include "base/string_number_conversions.h"
 #include "base/string_util.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_window.h"
 #include "chrome/browser/ui/window_snapshot/window_snapshot.h"
 #include "chrome/common/chrome_paths.h"
 #include "chrome/common/chrome_version_info.h"
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/test_launcher_utils.h"
+#include "chrome/test/base/tracing.h"
 #include "chrome/test/base/ui_test_utils.h"
 #include "content/public/browser/render_view_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/common/content_switches.h"
 #include "googleurl/src/gurl.h"
 #include "net/base/net_util.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/gfx/codec/png_codec.h"
@@ skipping 81 matching lines @@
   }
 
   // If the existing ref image was saved from an revision older than the
   // ref_img_update_revision, refresh the ref image.
   void RunPixelTest(const gfx::Size& tab_container_size,
                     const FilePath& url,
                     int64 ref_img_update_revision) {
     ref_img_revision_no_older_than_ = ref_img_update_revision;
     ObtainLocalRefImageFilePath();
 
+#if defined(OS_WIN)
+    ASSERT_TRUE(tracing::BeginTracing("-test_*"));
+#endif
+
     ASSERT_TRUE(ui_test_utils::BringBrowserWindowToFront(browser()));
 
     ui_test_utils::DOMMessageQueue message_queue;
     ui_test_utils::NavigateToURL(browser(), net::FilePathToFileURL(url));
 
     // Wait for notification that page is loaded.
     ASSERT_TRUE(message_queue.WaitForMessage(NULL));
     message_queue.ClearQueue();
 
     ASSERT_TRUE(ui_test_utils::ExecuteJavaScript(
@@ skipping 12 matching lines @@
     }
 
     bool ignore_bottom_corners = false;
 #if defined(OS_MACOSX)
     // On Mac Lion, bottom corners have shadows with random pixels.
     ignore_bottom_corners = true;
 #endif
 
     SkBitmap bitmap;
     ASSERT_TRUE(TabSnapShotToImage(&bitmap));
-    ASSERT_TRUE(CompareImages(bitmap, ignore_bottom_corners));
+    bool is_image_same = CompareImages(bitmap, ignore_bottom_corners);
+    EXPECT_TRUE(is_image_same);
+
+#if defined(OS_WIN)
+    std::string trace_events;
+    ASSERT_TRUE(tracing::EndTracing(&trace_events));
+    if (!is_image_same)
+      fprintf(stderr, "\n\nTRACE JSON:\n\n%s\n\n", trace_events.c_str());

[Zhenyao Mo, 2012/03/10 01:12:51]

It's worth adding a comment that this is for temporary debugging purpose. 
Otherwise such kind of code will freak people out :)

[jbates, 2012/03/12 17:32:11]

Done.

+#endif
   }
 
   const FilePath& test_data_dir() const {
     return test_data_dir_;
   }
 
  private:
   FilePath test_data_dir_;
   FilePath generated_img_dir_;
   FilePath ref_img_dir_;
@@ skipping 282 matching lines @@
 IN_PROC_BROWSER_TEST_F(Canvas2DPixelTestSD, MAYBE_Canvas2DRedBoxSD) {
   // If test baseline needs to be updated after a given revision, update the
   // following number. If no revision requirement, then 0.
   const int64 ref_img_revision_update = 123489;
 
   gfx::Size container_size(400, 300);
   FilePath url =
       test_data_dir().AppendASCII("pixel_canvas2d.html");
   RunPixelTest(container_size, url, ref_img_revision_update);
 }