Returns empty dict for GetIOStats on CrOS platforms.

tools/telemetry/telemetry/core/platform/cros_platform_backend.py

Index: tools/telemetry/telemetry/core/platform/cros_platform_backend.py
diff --git a/tools/telemetry/telemetry/core/platform/cros_platform_backend.py b/tools/telemetry/telemetry/core/platform/cros_platform_backend.py
index 147517220e8abacec9aa271726e4b0748fbc2ac2..68ecc34a4ad816f469dbd3112603b7cdd5bf7869 100644
--- a/tools/telemetry/telemetry/core/platform/cros_platform_backend.py
+++ b/tools/telemetry/telemetry/core/platform/cros_platform_backend.py
@@ -35,5 +35,10 @@ class CrosPlatformBackend(linux_platform_backend.LinuxPlatformBackend):
     except AssertionError:
       return ''
 
+  def GetIOStats(self, pid):
+    # There is no '/proc/<pid>/io' file on CrOS platforms
+    # Returns empty dict as it does in PlatformBackend.
+    return {}
+
   def GetOSName(self):
     return 'chromeos'