Persist setup metrics and have Chrome report them during UMA upload.

base/metrics/histogram_samples.cc

Index: base/metrics/histogram_samples.cc
diff --git a/base/metrics/histogram_samples.cc b/base/metrics/histogram_samples.cc
index 4cd8e556b81c556dff9c2050348c47449c2d06c9..80b7b72e531d8cc98b56f3fe48a96d39c800eb59 100644
--- a/base/metrics/histogram_samples.cc
+++ b/base/metrics/histogram_samples.cc
@@ -73,7 +73,8 @@ HistogramSamples::HistogramSamples(uint64_t id)
 HistogramSamples::HistogramSamples(uint64_t id, Metadata* meta)
     : meta_(meta) {
   DCHECK(meta_->id == 0 || meta_->id == id);
-  meta_->id = id;
+  if (!meta_->id)

[Alexei Svitkine (slow), 2016/02/19 16:57:00]

Nit: == 0, since it's a number.

However, I don't understand why this if is needed. If the above DCHECK is always
true, shouldn't the if be a no-op? Or is |meta_| read-only or something? If it's
something like the latter, needs a comment.

[bcwhite, 2016/02/19 19:50:14]

Done.

+    meta_->id = id;
 }
 
 HistogramSamples::~HistogramSamples() {}