Move arc_metrics_constants.h into separate source set

Enables other components to depend on this file instead of the entire
ARC library for UMA recording.

Required for upcoming change in //ash/system/message_center/arc:arc
to record user interactions to UMA.

Bug: 862001
Change-Id: I3100879344a95313963e9b2bf1861a8f8206a416
Reviewed-on: https://chromium-review.googlesource.com/1212429Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Shao-Chuan Lee <shaochuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589444}

components/arc/BUILD.gn

@@ -39,7 +39,6 @@ static_library("arc") {
     "intent_helper/open_url_delegate.h",
     "lock_screen/arc_lock_screen_bridge.cc",
     "lock_screen/arc_lock_screen_bridge.h",
-    "metrics/arc_metrics_constants.h",
     "metrics/arc_metrics_service.cc",
     "metrics/arc_metrics_service.h",
     "midis/arc_midis_bridge.cc",
@@ -71,6 +70,7 @@ static_library("arc") {
 
   public_deps = [
     ":arc_base",
+    ":arc_metrics_constants",
     ":prefs",
   ]
 
@@ -273,6 +273,12 @@ static_library("arc_test_support") {
   ]
 }
 
+source_set("arc_metrics_constants") {
+  sources = [
+    "metrics/arc_metrics_constants.h",
+  ]
+}
+
 source_set("unit_tests") {
   testonly = true
   sources = [