Adds ability to specify how aura::Env is created in content

content/public/app/BUILD.gn

Index: content/public/app/BUILD.gn
diff --git a/content/public/app/BUILD.gn b/content/public/app/BUILD.gn
index 620f170618c97393351e840fd0f89a7ac6ed6ef0..d09712d3138203415395d8d547072548ed798b47 100644
--- a/content/public/app/BUILD.gn
+++ b/content/public/app/BUILD.gn
@@ -21,6 +21,7 @@
 #       //content/public/app:both_sources (source set)
 
 import("//build/config/chrome_build.gni")
+import("//build/config/ui.gni")
 import("//services/service_manager/public/service_manifest.gni")
 
 public_app_shared_sources = [
@@ -42,6 +43,10 @@ public_app_shared_deps = [
   "//content/public/utility:utility_sources",
 ]
 
+if (use_aura) {
+  public_app_shared_deps += [ "//ui/aura" ]
+}
+
 if (is_component_build) {
   source_set("both_sources") {
     # Only the main content shared library can pull this in.