Add comment for BrowserChangeObserver

BrowserChangeObserver was just moved to be a publicly available class,
so this change adds a comment regarding how it should be used (or not
used, as the case may be).

Bug: 1052096
Change-Id: I7cdff02f7e2d87b65f17a61600a54e336b0671cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149913Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Cliff Smolinsky <cliffsmo@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#759286}

chrome/test/base/ui_test_utils.h

@@ -350,6 +350,8 @@ class HistoryEnumerator {
   DISALLOW_COPY_AND_ASSIGN(HistoryEnumerator);
 };
 
+// In general, tests should use WaitForBrowserToClose() and
+// WaitForBrowserToOpen() rather than instantiating this class directly.
 class BrowserChangeObserver : public BrowserListObserver {
  public:
   enum class ChangeType {