Set exp id in the CP header if user is in Lo-Fi control.

components/data_reduction_proxy/core/browser/data_reduction_proxy_config.cc

Index: components/data_reduction_proxy/core/browser/data_reduction_proxy_config.cc
diff --git a/components/data_reduction_proxy/core/browser/data_reduction_proxy_config.cc b/components/data_reduction_proxy/core/browser/data_reduction_proxy_config.cc
index df78cf8e6d81a0b615fa2a279522761ae94e5c67..c24213a709f610bc9c564d8644e9144325efaf6e 100644
--- a/components/data_reduction_proxy/core/browser/data_reduction_proxy_config.cc
+++ b/components/data_reduction_proxy/core/browser/data_reduction_proxy_config.cc
@@ -493,6 +493,24 @@ bool DataReductionProxyConfig::ShouldUseLoFiHeaderForRequests() const {
   return ShouldUseLoFiHeaderForRequests(lofi_status_);
 }
 
+bool DataReductionProxyConfig::IsInLoFiControlExperiment() const {
+  switch (lofi_status_) {
+    case LOFI_STATUS_OFF:
+    case LOFI_STATUS_TEMPORARILY_OFF:
+    case LOFI_STATUS_INACTIVE_CONTROL:
+    case LOFI_STATUS_INACTIVE:
+    case LOFI_STATUS_ACTIVE_FROM_FLAGS:
+    case LOFI_STATUS_ACTIVE:
+      return false;
+    case LOFI_STATUS_ACTIVE_CONTROL:
+      DCHECK(IsIncludedInLoFiControlFieldTrial());
+      return true;
+    default:
+      NOTREACHED() << lofi_status_;
+  }
+  return false;
+}
+
 void DataReductionProxyConfig::PopulateAutoLoFiParams() {
   std::string field_trial = params::GetLoFiFieldTrialName();