Add Ozone configuration to facilitate management of external platforms

ui/ozone/ozone_extra.gni

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# This list contains the name of external platforms that are passed to the
+# --ozone-platform command line argument or used for the ozone_platform build
+# config. For example ozone_external_platforms = [ "foo1", "foo2", ... ]

[rjkroege, 2016/10/31 20:41:46]

Could you add a realistic example -- as in what happens with CrOS?

[fwang, 2016/10/31 20:56:05]

Not sure I follow the connection with CrOS... These are just a list of Ozone
platforms and should be independent of the target OS.

+ozone_external_platforms = []
+
+# This list contains dependencies for external platforms. Typically, the Ozone
+# implementations are placed into ui/ozone/platform/ and so this will look
+# something like:
+# ozone_external_platform_deps = [ "platform/foo1", "platform/foo_2", ... ]
+ozone_external_platform_deps = []
+
+# If a platform has unit tests, the corresponding source_set can be listed here
+# so that they get included into ozone_unittests.
+# ozone_external_platform_test_deps = [ "platform/foo1:foo1_unitests", ... ]
+ozone_external_platform_test_deps = []