Add a Rappor option in the settings page.

components/rappor/rappor_pref_names.cc

Index: components/rappor/rappor_pref_names.cc
diff --git a/components/rappor/rappor_pref_names.cc b/components/rappor/rappor_pref_names.cc
index b879c4bd2b3ad13c33faee5e86914e0204bf35f0..1664f9a8613daa62294d22e6e3d19f26d420fa59 100644
--- a/components/rappor/rappor_pref_names.cc
+++ b/components/rappor/rappor_pref_names.cc
@@ -13,6 +13,9 @@ const char kRapporCohortDeprecated[] = "rappor.cohort";
 // A randomly generated number, which determines cohorts data is reported for.
 const char kRapporCohortSeed[] = "rappor.cohort_seed";
 
+// True if rappor reports should be uploaded.
+const char kRapporEnabled[] = "rappor.enabled";
+
 // Timestamp of the last time we sampled daily metrics.
 const char kRapporLastDailySample[] = "rappor.last_daily_sample";