[Clean up] Move VariationsHttpHeaderProvider into a subdirectory.

components/variations/BUILD.gn

Index: components/variations/BUILD.gn
diff --git a/components/variations/BUILD.gn b/components/variations/BUILD.gn
index 07b17e7890f4a9611bb92877eff30320cdb22f3a..d49328ef960b8f6522489ec73764d513eb1a1847 100644
--- a/components/variations/BUILD.gn
+++ b/components/variations/BUILD.gn
@@ -6,7 +6,7 @@ if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
-static_library("variations") {
+source_set("variations") {
   sources = [
     "active_field_trials.cc",
     "active_field_trials.h",
@@ -32,8 +32,6 @@ static_library("variations") {
     "study_filtering.h",
     "variations_associated_data.cc",
     "variations_associated_data.h",
-    "variations_http_header_provider.cc",
-    "variations_http_header_provider.h",
     "variations_seed_processor.cc",
     "variations_seed_processor.h",
     "variations_seed_simulator.cc",
@@ -41,8 +39,8 @@ static_library("variations") {
   ]
 
   deps = [
+    "proto",
     "//base",
-    "//components/variations/proto",
     "//third_party/mt19937ar",
   ]
 
@@ -60,3 +58,26 @@ if (is_android) {
     jni_package = "variations"
   }
 }
+
+source_set("unit_tests") {
+  testonly = true
+  sources = [
+    "active_field_trials_unittest.cc",
+    "caching_permuted_entropy_provider_unittest.cc",
+    "entropy_provider_unittest.cc",
+    "metrics_util_unittest.cc",
+    "net/variations_http_header_provider_unittest.cc",
+    "study_filtering_unittest.cc",
+    "variations_associated_data_unittest.cc",
+    "variations_seed_processor_unittest.cc",
+    "variations_seed_simulator_unittest.cc",
+  ]
+
+  deps = [
+    ":variations",
+    "net:variations_http_provider",
+    "proto",
+    "//base/test:test_support",
+    "//testing/gtest",
+  ]
+}