[sensors] Add Generic Sensor platform unit tests.

device/generic_sensor/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")  # For generate_jni().
 }
 
@@ skipping 43 matching lines @@
 
   android_library("java") {
     java_files = device_sensors_jni_sources
 
     deps = [
       "//base:base_java",
       "//device/generic_sensor/public/interfaces:interfaces_java",
     ]
   }
 }
+
+static_library("testing") {
+  testonly = true
+  sources = [
+    "fake_platform_sensor.cc",
+    "fake_platform_sensor.h",
+    "fake_platform_sensor_provider.cc",
+    "fake_platform_sensor_provider.h",
+  ]
+
+  public_deps = [
+    ":generic_sensor",
+    "//base",
+    "//device/generic_sensor/public/interfaces",
+  ]
+}