Reland of [sensors] Ambient light sensor implementation for ChromeOS and Linux.

device/generic_sensor/platform_sensor_provider.cc

 // 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.
 
 #include "device/generic_sensor/platform_sensor_provider.h"
 
 #if defined(OS_MACOSX)
 #include "device/generic_sensor/platform_sensor_provider_mac.h"
 #elif defined(OS_ANDROID)
 #include "device/generic_sensor/platform_sensor_provider_android.h"
 #elif defined(OS_WIN)
 #include "device/generic_sensor/platform_sensor_provider_win.h"
+#elif defined(OS_LINUX)
+#include "device/generic_sensor/platform_sensor_provider_linux.h"
 #endif
 
 namespace {
 
 static device::PlatformSensorProvider* g_provider_for_testing = nullptr;
 
 }  // namespace
 
 namespace device {
 
 // static
 void PlatformSensorProvider::SetProviderForTesting(
     PlatformSensorProvider* provider) {
   g_provider_for_testing = provider;
 }
 
 // static
 PlatformSensorProvider* PlatformSensorProvider::GetInstance() {
   if (g_provider_for_testing)
     return g_provider_for_testing;
 #if defined(OS_MACOSX)
   return PlatformSensorProviderMac::GetInstance();
 #elif defined(OS_ANDROID)
   return PlatformSensorProviderAndroid::GetInstance();
 #elif defined(OS_WIN)
   return PlatformSensorProviderWin::GetInstance();
-#else
-  return nullptr;
+#elif defined(OS_LINUX)
+  return PlatformSensorProviderLinux::GetInstance();
 #endif
 }
 
 }  // namespace device