Fix memory leak in ThreadWatcher.

chrome/browser/metrics/thread_watcher.cc

Index: chrome/browser/metrics/thread_watcher.cc
diff --git a/chrome/browser/metrics/thread_watcher.cc b/chrome/browser/metrics/thread_watcher.cc
index 84a00ae4e45f32c74fe06ff11da31ca4f7e64500..6d1ecda6cb5519103f7d88022932866eb0182c7c 100644
--- a/chrome/browser/metrics/thread_watcher.cc
+++ b/chrome/browser/metrics/thread_watcher.cc
@@ -84,8 +84,10 @@ void ThreadWatcher::StartWatching(const WatchingParams& params) {
   DCHECK(watcher);
   // If we couldn't register the thread watcher object, we are shutting down,
   // then don't activate thread watching.
-  if (!ThreadWatcherList::IsRegistered(params.thread_id))
+  if (!ThreadWatcherList::IsRegistered(params.thread_id)) {
+    delete watcher;

[Ilya Sherman, 2016/08/30 23:51:57]

Could you instead modify |watcher| to be a unique_ptr, and call
ThreadWatcherList::Register() immediately after constructing the watcher, rather
than as part of its constructor?  Moreover, if you update
ThreadWatcherList::Register() to return a boolean, I think you can get rid of
ThreadWatcherList::IsRegistered() and ThreadWatcherList::Find().  I think the
net result would be less, and easier-to-follow, code =)

     return;
+  }
   watcher->ActivateThreadWatching();

[alokp, 2016/08/30 22:32:34]

Is this leaked?

[ramant (doing other things), 2016/08/30 22:45:12]

watcher is stored in g_thread_watcher_list_. It is deleted in
ThreadWatcherList::DeleteAll().

IsRegistered returns false if g_thread_watcher_list_ is null.

This is the case when browser was shutting down even before we started watching
threads. 

We wanted threads to stabilize during browser startup (they would be busy and
threads may not be very responsive) before we start watching them (I think it 2
minutes after startup). 

If browser were to shutdown within 2 minutes and if g_thread_watcher_list_is
already deleted, then the above case could occur.

Writing from memory (so, I could be wrong).

[alokp, 2016/08/30 22:47:21]

Thanks for the explanation. I think a comment about this would be nice.

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