Update AccelerometerReader Error Output

Change log level from ERROR to WARNING so as to reduce confusion in testing bot logs.

TEST=manual testing on device
BUG=585508

Review URL: https://codereview.chromium.org/1687693003

Cr-Commit-Position: refs/heads/master@{#374698}

chromeos/accelerometer/accelerometer_reader.cc

@@ -235,8 +235,8 @@ void AccelerometerFileReader::Initialize(
   // file on detecting the device.
   if (base::IsDirectoryEmpty(base::FilePath(kAccelerometerDevicePath))) {
     if (base::SysInfo::IsRunningOnChromeOS()) {
-      LOG(ERROR) << "Accelerometer device directory is empty at "
-                 << kAccelerometerDevicePath;
+      LOG(WARNING) << "Accelerometer device directory is empty at "
+                   << kAccelerometerDevicePath;
     }
     return;
   }