Fix a wrong dependency in ash/BUILD.gn

BUG=None
R=cmasone@chromium.org
TBR=jamescook@chromium.org
TEST=gn gen

Review URL: https://codereview.chromium.org/586213002

Cr-Commit-Position: refs/heads/master@{#296031}

ash/BUILD.gn

@@ -356,7 +356,7 @@ executable("ash_shell") {
 
   if (is_chromeos) {
     deps += [
-      "../device/bluetooth/bluetooth.gyp:device_bluetooth",
+      "//device/bluetooth",
     ]
   }
 }