GitLab

avoids repainting stats graphs that are not visible.
Visibility is determined as described in
  https://stackoverflow.com/a/21696585/4352772

BUG=678292

Change-Id: Ib7ac623f8d961359e4915b3f5580734fb1328bec
Reviewed-on: https://chromium-review.googlesource.com/602270Reviewed-by: Tommi <tommi@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543372}