sensors: Add USE_IIOSERVICE build flag

The flag indicates if IIO Service is running in ChromeOS, which
determines sensor clients to use Mojo implementations or not.

BUG=b:173485266, b:168434557, b:172414302

Change-Id: Ib89dea49b03671e9ba36f19720c30e6035521731
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624327
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843397}

chromeos/components/sensors/BUILD.gn

@@ -2,8 +2,17 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
+import("//chromeos/components/sensors/buildflags.gni")
+
 assert(is_chromeos, "Non-ChromeOS builds cannot depend on //chromeos")
 
+buildflag_header("buildflags") {
+  header = "buildflags.h"
+
+  flags = [ "USE_IIOSERVICE=$use_iioservice" ]
+}
+
 component("sensors") {
   output_name = "chromeos_sensors"
   defines = [ "IS_CHROMEOS_SENSORS_IMPL" ]

chromeos/components/sensors/buildflags.gni

+# Copyright 2021 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.
+
+declare_args() {
+  # Indicates if iioservice daemon is installed.
+  use_iioservice = false
+}