Fix ozone_extra import issues

ui/ozone/ozone.gni was moved to build/config/ozone.gni, however, it
turns out that ozone.gni depends on ozone_extra.gni, so this CL moves
ui/ozone/ozone_extra.gni to build/config/ozone_extra.gni in order to
keep build/config self-contained.

Bug: 1129223
Change-Id: I0b67cbd41d7b402dfd595204a3a599053c83ba77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2451872
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#814305}

build/config/OWNERS

@@ -3,3 +3,4 @@ scottmg@chromium.org
 
 per-file *jumbo*=bratell.d@gmail.com
 per-file ozone.gni=file://ui/ozone/OWNERS
+per-file ozone_extra.gni=file://ui/ozone/OWNERS

build/config/ozone.gni

@@ -9,7 +9,7 @@ import("//build/toolchain/toolchain.gni")
 declare_args() {
   # Ozone extra platforms file path. Can be overridden to build out of
   # tree ozone platforms.
-  ozone_extra_path = "//ui/ozone/ozone_extra.gni"
+  ozone_extra_path = "//build/config/ozone_extra.gni"
 
   # Select platforms automatically. Turn this off for manual control.
   ozone_auto_platforms = use_ozone