Remove unused histograms from the background loader offliner.

Bug: 975512
Change-Id: I87b0a91bed60e3a9e8a1fd9ae9b18cac27a0859f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1683361
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Peter Williamson <petewil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#675332}
  content::WebContents* web_contents() { return stub_->web_contents(); }

  content::WebContents* web_contents() { return stub_->web_contents(); }
