Move ENABLE_HIDPI to new build flags system

tools/gn/docs/cookbook.md

 # GYP->GN Conversion Cookbook
 
 [TOC]
 
 ## Targets
 
 | *GYP*                                            | *GN*                                               |
 |:-------------------------------------------------|:---------------------------------------------------|
 | `'type': 'static_library', 'name': 'foo',`       | `static_library("foo") {` or `source_set("foo") {` |
 | `'type': 'shared_library', 'name': 'foo',`       | `shared_library("foo") {`                          |
@@ skipping 223 matching lines @@
 | `debug_devtools` (0/1)                  | `debug_devtools` (true/false)                  | `//build/config/features.gni` |
 | `disable_ftp_support` (0/1)             | `disable_ftp_support` (true/false)             | `//build/config/features.gni` |
 | `disable_nacl` (0/1)                    | `enable_nacl` (true/false)                     | `//build/config/features.gni` |
 | `enable_app_list` (0/1)                 | `enable_app_list` (true/false)                 | `//build/config/features.gni` |
 | `enable_autofill_dialog` (0/1)          | `enable_autofill_dialog` (true/false)          | `//build/config/features.gni` |
 | `enable_background` (0/1)               | `enable_background` (true/false)               | `//build/config/features.gni` |
 | `enable_captive_portal_detection` (0/1) | `enable_captive_portal_detection` (true/false) | `//build/config/features.gni` |
 | `enable_chromevox_next` (0/1)           | `enable_chromevox_next` (true/false)           | `//build/config/features.gni` |
 | `enable_extensions` (0/1)               | `enable_extensions` (true/false)               | `//build/config/features.gni` |
 | `enable_google_now` (0/1)               | `enable_google_now` (true/false)               | `//build/config/features.gni` |
-| `enable_hidpi` (0/1)                    | `enable_hidpi` (true/false)                    | `//build/config/ui.gni`       |
+| `enable_hidpi` (0/1)                    | `enable_hidpi` (true/false)                    | `//ui/base/ui_features.gni`   |
 | `enable_managed_users` (0/1)            | `enable_managed_users` (true/false)            | `//build/config/features.gni` |
 | `enable_mdns` (0/1)                     | `enable_mdns` (true/false)                     | `//build/config/features.gni` |
 | `enable_one_click_signin` (0/1)         | `enable_one_click_signin` (true/false)         | `//chrome/common/features.gni` |
 | `enable_pepper_cdms` (0/1)              | `enable_pepper_cdms` (true/false)              | `//build/config/features.gni` |
 | `enable_plugins` (0/1)                  | `enable_plugins` (true/false)                  | `//build/config/features.gni` |
 | `enable_plugin_installation` (0/1)      | `enable_plugin_installation` (true/false)      | `//build/config/features.gni` |
 | `enable_basic_printing` (0/1)           | `enable_basic_printing` (true/false)           | `//build/config/features.gni` |
 | `enable_print_preview` (0/1)            | `enable_print_preview` (true/false)            | `//build/config/features.gni` |
 | `enable_rlz` (0/1)                      | `enable_rlz` (true/false)                      | `//build/config/features.gni` |
 | `enable_service_discovery` (0/1)        | `enable_service_discovery` (true/false)        | `//build/config/features.gni` |
@@ skipping 415 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```