Fix lowPowerModeEnabled crash on iOS8

BUG=webrtc::5564

Review URL: https://codereview.webrtc.org/1739893003

Cr-Commit-Position: refs/heads/master@{#11774}
diff --git a/webrtc/modules/utility/source/helpers_ios.mm b/webrtc/modules/utility/source/helpers_ios.mm
index 4b28cee..a312f03 100644
--- a/webrtc/modules/utility/source/helpers_ios.mm
+++ b/webrtc/modules/utility/source/helpers_ios.mm
@@ -170,7 +170,12 @@
 }
 
 bool GetLowPowerModeEnabled() {
-  return [NSProcessInfo processInfo].lowPowerModeEnabled;
+  NSProcessInfo* info = [NSProcessInfo processInfo];
+  // lowPoweredModeEnabled is only available on iOS9+.
+  if ([info respondsToSelector:@selector(lowPoweredModeEnabled)]) {
+    return info.lowPowerModeEnabled;
+  }
+  return false;
 }
 
 }  // namespace ios