Add GN build file for ui/aura target.

ui/events/platform/BUILD.gn

Index: ui/events/platform/BUILD.gn
diff --git a/ui/events/platform/BUILD.gn b/ui/events/platform/BUILD.gn
index e84299303b67a57d77dd45d58b2c940e1ee38adc..3a20219a33cc374d8a1be0d830cf2d92bb2bb08b 100644
--- a/ui/events/platform/BUILD.gn
+++ b/ui/events/platform/BUILD.gn
@@ -5,14 +5,6 @@
 import("//build/config/ui.gni")
 
 component("platform") {
-  deps = [
-    "//base"
-  ]
-
-  defines = [
-    "EVENTS_IMPLEMENTATION",
-  ]
-
   sources = [
     "platform_event_dispatcher.h",
     "platform_event_observer.h",
@@ -24,7 +16,17 @@ component("platform") {
     "scoped_event_dispatcher.h",
   ]
 
+  defines = [
+    "EVENTS_IMPLEMENTATION",
+  ]
+
+  deps = [
+    "//base"
+  ]
+
   if (use_x11) {
-    sources -= [ "platform_event_source_stub.cc" ]
+    sources -= [
+      "platform_event_source_stub.cc",
+    ]
   }
 }