Add what/why/how-to-avoid statements to the provider of user demographics.

Give more information about what/why demographic data is used and how to avoid reporting that data.

Bug: 990822
Change-Id: I79237d6f5ce0522709a311a86f57e1e855881fac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735975
Commit-Queue: Vincent Boisselle <vincb@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684133}

components/metrics/demographic_metrics_provider.h

@@ -18,6 +18,10 @@ struct Feature;
 
 namespace metrics {
 
+// Provider of user demographics. Aggregate, non-individually-identifying
+// demographics are used to measure usage of Chrome features by age and gender.
+// Users can avoid aggregation of usage data by demographics by either a)
+// turning off sending usage statistics to Google or b) turning off sync.
 class DemographicMetricsProvider : public MetricsProvider {
  public:
   // Interface that represents the client that retrieves Profile information.