Move FileEnumerator to its own file, do some refactoring.

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 # running overnight and removing outdated bugs/suppressions.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 577 matching lines @@
 }
 {
    bug_56676_b
    Heapcheck:Leak
    fun:__gnu_cxx::new_allocator::allocate
    fun:std::string::_Rep::_S_create
    fun:std::string::_S_construct
    fun:std::string::_S_construct_aux
    fun:std::string::_S_construct
    fun:*basic_string*
-   fun:file_util::FileEnumerator::ReadDirectory
-   fun:file_util::FileEnumerator::Next
+   fun:base::FileEnumerator::ReadDirectory
+   fun:base::FileEnumerator::Next
    fun:::RelayReadDirectory::RunWork
    fun:::MessageLoopRelay::ProcessOnTargetThread
 }
 {
    bug_58474_a
    Heapcheck:Leak
    ...
    fun:FileSystemHostContextTest_CheckValidPath_Test::TestBody
 }
 {
@@ skipping 1233 matching lines @@
    ...
    fun:DiskCacheTestWithCache::InitCache
    fun:DiskCacheBackendTest_SimpleDoom*
 }
 {
    bug_236791
    Heapcheck:Leak
    ...
    fun:ash::test::FocusCyclerTest_CycleFocusThroughWindowWithPanes_Test::TestBody
 }