//chromeos GN cleanup

This CL:
* Moves system/devicetype to chromeos/constants to reduce dependencies
  for code that includes devicetype.h (which onbly depends on //base).
* Removes dependencies on //chromeos that are no longer necessary.
* Updates //chromeos and //chromeos/dbus dependencies in test targets
  to use :test_support.

For straightforward deps removal and file renames in:
    components/exo
    components/metrics
    components/pairing
    components/rlz
    components/sync
    content/shell
    device/bluetooth/bluez
    device/usb
    extensions/shell
    media/audio
    ui/keyboard
[email protected]

Bug: 918682
Change-Id: I4c83b5703303fc01f64d21c43a4b9b6690c79fef
Reviewed-on: https://chromium-review.googlesource.com/c/1407388
Commit-Queue: Steven Bennetts <[email protected]>
Reviewed-by: Jacob Dufault <[email protected]>
Reviewed-by: Ryo Hashimoto <[email protected]>
Cr-Commit-Position: refs/heads/master@{#623018}
diff --git a/content/browser/BUILD.gn b/content/browser/BUILD.gn
index d3ab894..02b749a 100644
--- a/content/browser/BUILD.gn
+++ b/content/browser/BUILD.gn
@@ -2414,7 +2414,7 @@
 
   if (is_chromeos) {
     deps += [
-      "//chromeos",
+      "//chromeos:chromeos_constants",
       "//chromeos/assistant:buildflags",
       "//chromeos/audio",
       "//chromeos/dbus",