Disable flaky test on Mac

The test has been flaky recently. Suspecting crrev.com/c/1686728. See
bug for details.

TBR=rsesek@chromium.org
TBR=mark@chromium.org

Bug: 986288
Change-Id: I6ddcf1b675871db30e786c0e152debe1a8fdd786
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712048Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679547}

content/browser/child_process_task_port_provider_mac_unittest.cc

@@ -173,7 +173,8 @@ TEST_F(ChildProcessTaskPortProviderTest, ChildLifecycle) {
   EXPECT_EQ(1u, GetDeadNameRefCount(send_right.get()));
 }
 
-TEST_F(ChildProcessTaskPortProviderTest, DeadTaskPort) {
+// Test is flaky. See https://crbug.com/986288.
+TEST_F(ChildProcessTaskPortProviderTest, DISABLED_DeadTaskPort) {
   EXPECT_EQ(kMachPortNull, provider()->TaskForPid(6));
 
   // Create a fake task port for the fake process.