Suppress the heapcheck leak in a test.

tools/heapcheck/suppressions.txt

 # There are three kinds of suppressions in this file.
 # 1. third party stuff we have no control over
 #
 # 2. intentional unit test errors, or stuff that is somehow a false positive
 # in our own code, or stuff that is so trivial it's not worth fixing
 #
 # 3. Suppressions for real chromium bugs that are not yet fixed.
 # These should all be in chromium's bug tracking system (but a few aren't yet).
 # Periodically we should sweep this file and the bug tracker clean by
 # running overnight and removing outdated bugs/suppressions.
@@ skipping 305 matching lines @@
   fun:scoped_ptr::reset
   fun:ProfileSyncServiceTypedUrlTest::TearDown
 }
 
 {
   bug_38490
   Heapcheck:Leak
   ...
   fun:browser_sync::SyncBackendHost::Core::NotifyFrontend
 }
+
+{
+  bug_40499
+  Heapcheck:Leak
+  fun:UserScriptSlaveTest::Serialize
+  ...
+  fun:testing::Test::Run
+  fun:testing::internal::TestInfoImpl::Run
+  fun:testing::TestCase::Run
+  fun:testing::internal::UnitTestImpl::RunAllTests
+  fun:testing::UnitTest::Run
+  fun:TestSuite::Run
+}