[PM] Add a comment to clarify how to use PM in unit_tests

Change-Id: I9f9b477b2d2fbad916c6366315638576f08f87b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2292490Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Sigurður Ásgeirsson <siggi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787724}

components/performance_manager/test_support/performance_manager_test_harness.h

@@ -16,7 +16,7 @@ namespace performance_manager {
 // via content::RenderViewHostTestHarness::task_environment().
 //
 // Meant to be used from components_unittests, but not from unit_tests or
-// browser tests.
+// browser tests. unit_tests should use PerformanceManagerTestHarnessHelper.
 //
 // To set the active WebContents for the test use:
 //