Fix race condition in DataFetcherSharedMemoryBase

tools/valgrind/tsan/suppressions.txt

 # There are a few 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 # running overnight and removing outdated bugs/suppressions.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 1111 matching lines @@
 }
 {
   bug_268924
   ThreadSanitizer:Race
   fun:base::PowerMonitor::PowerMonitor
   fun:content::ChildThread::Init
   fun:content::ChildThread::ChildThread
   fun:content::UtilityThreadImpl::UtilityThreadImpl
   fun:content::UtilityMainThread::InitInternal
 }
-{
-   bug_284959
-   ThreadSanitizer:Race
-   fun:std::_Rb_tree_insert_and_rebalance
-   fun:std::_Rb_tree::_M_insert_
-   fun:std::_Rb_tree::_M_insert_unique_
-   fun:std::map::insert
-   fun:std::map::operator[]
-   fun:content::DataFetcherSharedMemoryBase::GetSharedMemory
-   fun:content::DataFetcherSharedMemoryBase::GetSharedMemoryBuffer
-   fun:content::DataFetcherSharedMemoryBase::StartFetchingDeviceData
-   fun:content::::DataFetcherSharedMemoryBaseTest_DoesPollMotionAndOrientation_Test::TestBody
-   fun:testing::internal::HandleSehExceptionsInMethodIfSupported
-}