[Mac] Reenable TaskManagerBrowserTest.IdleWakeup

The TaskManagerBrowserTest.IdleWakeup browser test was disabled for
flakiness. The issue is this test, written with Chrome Win in mind,
was expected to generate 1000s of idle wakes, but at least 100. However
on the Mac the idle wake count is lower on certain releases (e.g.
between 65 and 81 on macOS 10.12). This cl reenables the test on macOS
and lowers the expected wake count.

Bug: 636823
Change-Id: Idba223012e2e868c39bf2b27d02c167e3d3dbad2
Reviewed-on: https://chromium-review.googlesource.com/876920
Commit-Queue: Jayson Adams <shrike@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530700}

chrome/browser/task_manager/task_manager_browsertest.cc

@@ -750,14 +750,7 @@ IN_PROC_BROWSER_TEST_F(TaskManagerBrowserTest, TotalSentDataObserved) {
 }
 
 // Checks that task manager counts idle wakeups.
-// Flakily fails on Mac: http://crbug.com/639939
-#if defined(OS_MACOSX)
-#define MAYBE_IdleWakeups DISABLED_IdleWakeups
-#else
-#define MAYBE_IdleWakeups IdleWakeups
-#endif  // defined(OS_MACOSX)
-IN_PROC_BROWSER_TEST_F(TaskManagerBrowserTest,
-                       MAYBE_IdleWakeups) {
+IN_PROC_BROWSER_TEST_F(TaskManagerBrowserTest, IdleWakeups) {
   ShowTaskManager();
   model()->ToggleColumnVisibility(ColumnSpecifier::IDLE_WAKEUPS);
 
@@ -775,10 +768,17 @@ IN_PROC_BROWSER_TEST_F(TaskManagerBrowserTest,
       browser()->tab_strip_model()->GetActiveWebContents(), test_js, &ok));
   ASSERT_EQ("okay", ok);
 
-  // The script above should trigger a lot of idle wakeups - up to 1000 per
-  // second. Let's make sure we get at least 100 (in case the test runs slow).
+// The script above should trigger a lot of idle wakeups - up to 1000 per
+// second. Let's make sure we get at least 100 (in case the test runs slow).
+// On Mac, set a lower threshold because Chrome Mac generates fewer wakes.
+#if defined(OS_MACOSX)
+  const int kMinExpectedWakeCount = 50;
+#else
+  const int kMinExpectedWakeCount = 100;
+#endif  // defined(OS_MACOSX)
   ASSERT_NO_FATAL_FAILURE(WaitForTaskManagerStatToExceed(
-      MatchTab("title1.html"), ColumnSpecifier::IDLE_WAKEUPS, 100));
+      MatchTab("title1.html"), ColumnSpecifier::IDLE_WAKEUPS,
+      kMinExpectedWakeCount));
   ASSERT_NO_FATAL_FAILURE(WaitForTaskManagerRows(1, MatchAnyTab()));
   ASSERT_NO_FATAL_FAILURE(WaitForTaskManagerRows(1, MatchTab("title1.html")));
 }