Fix flakiness in TwoClientPrintersSyncTest.ConflictResolution with RunAllTasksUntilIdle().

This CL adds a call to RunAllTasksUntilIdle() between the 2 description changes
in TwoClientPrintersSyncTest.ConflictResolution. This mimics what was
done prior to https://chromium-review.googlesource.com/c/chromium/src/+/890000
and should eliminate flakiness.

Bug: 810408
Change-Id: I52b7bc3a0ac76b4b2ddb0881b60ff137ba93e2c8
Reviewed-on: https://chromium-review.googlesource.com/1165800Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582152}

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

@@ -125,6 +125,12 @@ IN_PROC_BROWSER_TEST_F(TwoClientPrintersSyncTest, ConflictResolution) {
   // Wait for a non-zero period (200ms).
   base::PlatformThread::Sleep(base::TimeDelta::FromMilliseconds(200));
 
+  // Run all tasks so the first description change is applied.
+  // TODO(crbug.com/810408): This is a temporary fix to prevent flakiness. Tasks
+  // shouldn't run between description changes, because it prevents a real
+  // conflict from happening.
+  content::RunAllTasksUntilIdle();
+
   ASSERT_TRUE(
       EditPrinterDescription(GetPrinterStore(0), 0, kLatestDescription));