Mark some disabled tests as flaky to increase test coverage.

net/disk_cache/backend_unittest.cc

Index: net/disk_cache/backend_unittest.cc
diff --git a/net/disk_cache/backend_unittest.cc b/net/disk_cache/backend_unittest.cc
index 5fc465342857c88104a1ca95dc5f6f936d5e4bd5..6609d3a949c0022520c9dd4b8bcd177c3276cbf9 100644
--- a/net/disk_cache/backend_unittest.cc
+++ b/net/disk_cache/backend_unittest.cc
@@ -1413,9 +1413,9 @@ TEST_F(DiskCacheBackendTest, DisableSuccess4) {
   BackendDisable4();
 }
 
-// Flaky, http://crbug.com/21110. Don't re-enable without a real fix or adding
-// more debugging info!
-TEST_F(DiskCacheBackendTest, DISABLED_NewEvictionDisableSuccess4) {
+// Flaky, http://crbug.com/21110.
+// TODO(rvargas): Add more debugging code to help identify the root cause.
+TEST_F(DiskCacheBackendTest, FLAKY_NewEvictionDisableSuccess4) {
   ASSERT_TRUE(CopyTestCache(L"bad_rankings"));
   DisableFirstCleanup();
   SetDirectMode();