[sensors] Introduce Generic Sensor API interfaces

device/sensors/BUILD.gn

Index: device/sensors/BUILD.gn
diff --git a/device/sensors/BUILD.gn b/device/sensors/BUILD.gn
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+# 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.
+
+source_set("sensors") {
+  sources = [
+    "sensor_export.h",
+    "sensor_service.cc",
+    "sensor_service.h",
+  ]
+
+  defines = [ "DEVICE_SENSORS_IMPLEMENTATION" ]
+
+  deps = [
+    "//base",
+    "//device/sensors/mojo",
+  ]
+}