Use content::RunBlockingPoolTask() in tests.

ash/desktop_background/desktop_background_controller_unittest.cc

Index: ash/desktop_background/desktop_background_controller_unittest.cc
diff --git a/ash/desktop_background/desktop_background_controller_unittest.cc b/ash/desktop_background/desktop_background_controller_unittest.cc
index 8751a18c4db1ff788b0e9fc0bb4daabb6ca68dd2..01a71594ee0e7b878bc3398fca5897a12adbaa82 100644
--- a/ash/desktop_background/desktop_background_controller_unittest.cc
+++ b/ash/desktop_background/desktop_background_controller_unittest.cc
@@ -291,8 +291,7 @@ TEST_F(DesktopBackgroundControllerTest, ResizeCustomWallpaper) {
   // that the resized image is the expected size.
   controller_->SetWallpaperImage(image, WALLPAPER_LAYOUT_STRETCH);
   EXPECT_TRUE(image.BackedBySameObjectAs(controller_->GetWallpaper()));
-  content::BrowserThread::GetBlockingPool()->FlushForTesting();
-  content::RunAllPendingInMessageLoop();
+  content::RunAllBlockingPoolTasksUntilIdle();
   gfx::ImageSkia resized_image = controller_->GetWallpaper();
   EXPECT_FALSE(image.BackedBySameObjectAs(resized_image));
   EXPECT_EQ(gfx::Size(320, 200).ToString(), resized_image.size().ToString());
@@ -301,8 +300,7 @@ TEST_F(DesktopBackgroundControllerTest, ResizeCustomWallpaper) {
   // previously-resized image instead of doing another resize
   // (http://crbug.com/321402).
   controller_->SetWallpaperImage(image, WALLPAPER_LAYOUT_STRETCH);
-  content::BrowserThread::GetBlockingPool()->FlushForTesting();
-  content::RunAllPendingInMessageLoop();
+  content::RunAllBlockingPoolTasksUntilIdle();
   EXPECT_TRUE(resized_image.BackedBySameObjectAs(controller_->GetWallpaper()));
 }