Remove TODOs in PersistentHistogramStorage dtor and unit_test

Change-Id: I58a6a878b511e9ab2c27088ffc0c8069cf39b473
Reviewed-on: https://chromium-review.googlesource.com/c/1347311Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Xi Cheng <chengx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611179}

base/metrics/persistent_histogram_storage.cc

@@ -39,8 +39,6 @@ PersistentHistogramStorage::~PersistentHistogramStorage() {
   PersistentHistogramAllocator* allocator = GlobalHistogramAllocator::Get();
   allocator->UpdateTrackingHistograms();
 
-  // TODO(chengx): Investigate making early return depend on whethere there are
-  // metrics to report at this point or not.
   if (disabled_)
     return;
 

base/metrics/persistent_histogram_storage_unittest.cc

@@ -50,9 +50,8 @@ class PersistentHistogramStorageTest : public testing::Test {
   DISALLOW_COPY_AND_ASSIGN(PersistentHistogramStorageTest);
 };
 
-// TODO(chengx): Re-enable the test on OS_IOS after crbug.com/836789 is fixed.
-// PersistentHistogramStorage is only used on OS_WIN now, so disabling this
-// test on OS_IOS is fine.
+// This test is disabled on OS_IOS because of crbug.com/836789. This is fine as
+// PersistentHistogramStorage is only used on OS_WIN now.
 #if !defined(OS_NACL) && !defined(OS_IOS)
 TEST_F(PersistentHistogramStorageTest, HistogramWriteTest) {
   auto persistent_histogram_storage =