Support experiment configurations with/without stability metrics.

components/metrics/file_metrics_provider.cc

Index: components/metrics/file_metrics_provider.cc
diff --git a/components/metrics/file_metrics_provider.cc b/components/metrics/file_metrics_provider.cc
index aa81a0cd1f02c55dd7255bc17711c9323213dafe..f7f0b5e949e491644fa4a995178b27d0e298a6a3 100644
--- a/components/metrics/file_metrics_provider.cc
+++ b/components/metrics/file_metrics_provider.cc
@@ -106,6 +106,8 @@ struct FileMetricsProvider::SourceInfo {
   DISALLOW_COPY_AND_ASSIGN(SourceInfo);
 };
 
+bool FileMetricsProvider::stability_metrics_enabled_ = true;
+
 FileMetricsProvider::FileMetricsProvider(
     const scoped_refptr<base::TaskRunner>& task_runner,
     PrefService* local_state)
@@ -470,6 +472,8 @@ void FileMetricsProvider::OnDidCreateMetricsLog() {
 
 bool FileMetricsProvider::HasInitialStabilityMetrics() {
   DCHECK(thread_checker_.CalledOnValidThread());
+  if (!stability_metrics_enabled_)

[Alexei Svitkine (slow), 2016/11/25 19:44:42]

Instead of the extra boolean, why not check the param directly here? (You can
use a helper function in the anon namespace to make the call site simple).

This way, it avoids extra complexity of a static and having to set it from
somewhere else.

[bcwhite, 2016/11/28 15:35:46]

I could.  I thought it would be better to keep experiment access in one place. 
There's nothing else in this class that knows about experiments.

[Alexei Svitkine (slow), 2016/11/28 16:08:49]

All other things being equal, I agree.

However, here it's a trade off between making the API surface more complex vs.
having the code to check experiment params be in separate files.

I think it would be better to keep the API surface better at the cost of
checking the params in two different places.

[bcwhite, 2016/11/29 19:09:07]

Done.

+    return false;
 
   // Measure the total time spent checking all sources as well as the time
   // per individual file. This method is called during startup and thus blocks