tracing: simplify lifetime of TraceEventFilter(s)

base/trace_event/event_name_filter.h

Index: base/trace_event/event_name_filter.h
diff --git a/base/trace_event/event_name_filter.h b/base/trace_event/event_name_filter.h
index 19333b3e03338d5d6981e71e0c4c3ce2b9073f26..87402ae716b1446764c68105a91f4dfb5a6df5a3 100644
--- a/base/trace_event/event_name_filter.h
+++ b/base/trace_event/event_name_filter.h
@@ -28,7 +28,8 @@ class BASE_EXPORT EventNameFilter : public TraceEventFilter {
   using EventNamesWhitelist = std::unordered_set<std::string>;
   static const char kName[];
 
-  EventNameFilter(std::unique_ptr<EventNamesWhitelist>);
+  EventNameFilter(std::unique_ptr<TraceEventFilter::Config>,
+                  std::unique_ptr<EventNamesWhitelist>);

[ssid, 2016/12/12 04:50:18]

I don't think this argument is needed anymore. the whitelist is taken from the
args in the config.
Maybe TraceEventFilter::Config::GetArgAsSet() should be part of the interface?

event_names_whitelist = filter_config.GetArgAsSet("event_name_whitelist",
&*whitelist)

[Primiano Tucci (use gerrit), 2016/12/12 14:38:55]

Nope I don't want for filters to depend on TraceConfig.
This is more about the split between the common part of tracing and the
chromium-specific. I have not clear yet where TraceConfig will end up, and in
doubt I don't want anything % TraceLog to depend on TraceConfig. This makes the
overall code easier to reason about.

From a concrete viewpoint also, we need a hashtable here, so we need to turn the
traceconfig into an unordered_dict at least once. I prefer then doing that into
TraceLog instead, to keep dependencies cleaner.

[ssid, 2016/12/12 21:25:53]

Yes it makes that we need a hashtable here for the strings. We could keep the
table separate.

It's just a bit weird that you need to pass the config to filters in some other
way. But, yes, this looks good now.

   ~EventNameFilter() override;
 
   // TraceEventFilter implementation.