Move the thread hop for UMA user actions from content:: to base::.

components/metrics/BUILD.gn

Index: components/metrics/BUILD.gn
diff --git a/components/metrics/BUILD.gn b/components/metrics/BUILD.gn
index 224070db0d464c758276cec3e7fdf0d47daee24c..b30f8986e5c635fd392ed916522248bd1c398b6f 100644
--- a/components/metrics/BUILD.gn
+++ b/components/metrics/BUILD.gn
@@ -257,6 +257,7 @@ if (!is_ios) {
 
 # GYP version: components/metrics.gypi:metrics_test_support
 source_set("test_support") {
+  testonly = true
   sources = [
     "test_metrics_provider.cc",
     "test_metrics_provider.h",
@@ -269,6 +270,7 @@ source_set("test_support") {
   ]
   deps = [
     "//base",
+    "//base/test:test_support",
   ]
 }