Flush UKM metrics when backgrounded on Android and iOS.

ios/chrome/test/testing_application_context.mm

Index: ios/chrome/test/testing_application_context.mm
diff --git a/ios/chrome/test/testing_application_context.mm b/ios/chrome/test/testing_application_context.mm
index 83102942cf09b228dbd965610e64c918e7c6ec2f..9d8eda29a6e49cf14c3e027d101662110bd9de8d 100644
--- a/ios/chrome/test/testing_application_context.mm
+++ b/ios/chrome/test/testing_application_context.mm
@@ -105,6 +105,11 @@ metrics::MetricsService* TestingApplicationContext::GetMetricsService() {
   return nullptr;
 }
 
+ukm::UkmService* TestingApplicationContext::GetUkmService() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  return nullptr;
+}
+
 variations::VariationsService*
 TestingApplicationContext::GetVariationsService() {
   DCHECK(thread_checker_.CalledOnValidThread());