PreloadCheck::ResultCallback: Avoid container copy in callback.

Bug: None
Change-Id: I7c510e133a6c580129030e3dd500c35aa0ee5767
Reviewed-on: https://chromium-review.googlesource.com/709934
Reviewed-by: Michael Giuffrida <[email protected]>
Commit-Queue: Istiaque Ahmed <[email protected]>
Cr-Commit-Position: refs/heads/master@{#507870}
diff --git a/extensions/browser/preload_check_group.h b/extensions/browser/preload_check_group.h
index 574f78b..92f48c38 100644
--- a/extensions/browser/preload_check_group.h
+++ b/extensions/browser/preload_check_group.h
@@ -32,7 +32,7 @@
 
  private:
   // Saves any errors and may invoke the callback.
-  virtual void OnCheckComplete(Errors errors);
+  virtual void OnCheckComplete(const Errors& errors);
 
   // Invokes the callback if the checks are considered finished.
   void MaybeInvokeCallback();