Move FileEnumerator to its own file, do some refactoring.

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 # running overnight and removing outdated bugs/suppressions.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 926 matching lines @@
   fun:::BrowserMainRunnerImpl::Initialize
   fun:BrowserMain
   fun:::RunNamedProcessTypeMain
   fun:::ContentMainRunnerImpl::Run
 }
 {
   bug_125928_b
   ThreadSanitizer:Race
   fun:__alloc_dir
   fun:opendir
-  fun:file_util::FileEnumerator::ReadDirectory
-  fun:file_util::FileEnumerator::Next
+  fun:base::FileEnumerator::ReadDirectory
+  fun:base::FileEnumerator::Next
   fun:::GetPrefsCandidateFilesFromFolder
   fun:ExternalPrefLoader::ReadStandaloneExtensionPrefFiles
   fun:ExternalPrefLoader::LoadOnFileThread
   fun:base::internal::RunnableAdapter::Run
 }
 {
   bug_121574
   ThreadSanitizer:Race
   fun:base::Thread::message_loop
   fun:content::BrowserThreadImpl::PostTaskHelper
@@ skipping 191 matching lines @@
   fun:KiUserCallbackDispatcher
   fun:LoadStringA
   fun:LoadStringA
   fun:LoadStringA
   fun:CreateWindowExW
   fun:base::MessagePumpForUI::InitMessageWnd
   fun:base::MessagePumpForUI::MessagePumpForUI
   fun:base::MessageLoop::MessageLoop
   fun:remoting::AutoThread::ThreadMain
 }