[TaskEnvironment] Migrate heap_collector_unittest.cc

Per https://chromium-review.googlesource.com/c/chromium/src/+/1756247/2/chrome/browser/metrics/perf/heap_collector_unittest.cc#695
this is migrated on its own because of the non-trivial requirement
to avoid formatting away intentional trailing whitespaces.

Edit: Turns out trailing whitespace formatting was my editor's fault
not git cl format. Simple one-line migration in latest patch set.

R=gmx@chromium.org

Bug: 992483
Change-Id: I8f6331ce2c187b41f1f541fbe03884a9138cfff1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1759145
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Marin <gmx@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688192}

chrome/browser/metrics/perf/heap_collector_unittest.cc

@@ -193,7 +193,7 @@ class HeapCollectorTest : public testing::Test {
  public:
   HeapCollectorTest()
       : test_browser_thread_bundle_(
-            base::test::ScopedTaskEnvironment::TimeSource::MOCK_TIME) {}
+            base::test::TaskEnvironment::TimeSource::MOCK_TIME) {}
 
   void SaveProfile(std::unique_ptr<SampledProfile> sampled_profile) {
     cached_profile_data_.resize(cached_profile_data_.size() + 1);