Enable printers sync_integration_tests on chromeos

chrome/browser/sync/test/integration/printers_helper.cc

Index: chrome/browser/sync/test/integration/printers_helper.cc
diff --git a/chrome/browser/sync/test/integration/printers_helper.cc b/chrome/browser/sync/test/integration/printers_helper.cc
index 084e1d02ae2293e88a6eed2792136d7bf88200b0..adc0c66a04b4b98120278a0550076c02323cf683 100644
--- a/chrome/browser/sync/test/integration/printers_helper.cc
+++ b/chrome/browser/sync/test/integration/printers_helper.cc
@@ -13,10 +13,12 @@
 #include "base/memory/ptr_util.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "chrome/browser/chromeos/printing/printers_manager.h"
 #include "chrome/browser/chromeos/printing/printers_manager_factory.h"
 #include "chrome/browser/sync/test/integration/sync_datatype_helper.h"
 #include "chrome/browser/sync/test/integration/sync_test.h"
+#include "content/public/browser/browser_thread.h"
 
 using sync_datatype_helper::test;
 
@@ -110,6 +112,9 @@ chromeos::PrintersManager* GetVerifierPrinterStore() {
       chromeos::PrintersManagerFactory::GetForBrowserContext(
           sync_datatype_helper::test()->verifier());
   // Must wait for ModelTypeStore initialization.
+  // Flush here since PrintersManagerFactory::BuildServiceInstanceFor() uses

[skym, 2017/04/06 16:48:44]

Can you expand on this comment. Why are we flushing? You're explaining why
you've chosen this particular BlockingPool, but not why flushing is
important/required.

[Gang Wu, 2017/04/06 19:22:59]

Done.

+  // content::BrowserThread::GetBlockingPool().
+  content::BrowserThread::GetBlockingPool()->FlushForTesting();

[skym, 2017/04/06 16:48:44]

I don't like how this is being done. Do we really need to flush/run until idle
before every PrintersManager access? Can this just be done once at the start of
each test case? Or even better, inside SetupSync()?

[Gang Wu, 2017/04/06 19:22:59]

Done.

   base::RunLoop().RunUntilIdle();
 
   return manager;
@@ -120,6 +125,9 @@ chromeos::PrintersManager* GetPrinterStore(int index) {
       chromeos::PrintersManagerFactory::GetForBrowserContext(
           sync_datatype_helper::test()->GetProfile(index));
   // Must wait for ModelTypeStore initialization.
+  // Flush here since PrintersManagerFactory::BuildServiceInstanceFor() uses
+  // content::BrowserThread::GetBlockingPool().
+  content::BrowserThread::GetBlockingPool()->FlushForTesting();
   base::RunLoop().RunUntilIdle();
 
   return manager;