blink scheduler: Add TimeSinceBackgrounded to RendererSchedulerTask UKM.

This was previously approved at [1] by msramek@.

[1] https://docs.google.com/document/d/14c_oWM2B02_mp43bPLcUiN6-094djfnzpbu9SAwH5-4/edit?ts=5a54f254#

Bug: 832162
Change-Id: I8f0f29af540a2e6f817b5e0b470683ad68a581b2
Reviewed-on: https://chromium-review.googlesource.com/1079068
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#563181}
2 files changed