Revert of Implement a site engagement score based on time-on-site.

Revert of Implement a site engagement score based on time-on-site. (patchset #20 id:400001 of https://codereview.chromium.org/1338603002/ )

Reason for revert:
Appears to break SiteEngagementServiceBrowserTest.SimulateInput on at least Mac 10.10: see https://build.chromium.org/p/chromium.mac/builders/Mac10.10%20Tests/builds/3034.

Original issue's description:
> Implement a site engagement score based on time-on-site.
>
> Time spent on site is an important indicator of how important an origin
> is to a user. This CL detects time on site by registering callbacks to
> handle user input, using any mouse or keypress events as a proxy for
> a user spending time on the site. Input events are checked for at
> regular intervals, with site engagement updated if input has been
> present in each interval.
>
> BUG=464234
>
> Committed: https://crrev.com/f3226a4f280b20eb00024d724b9e70ec1edc2e33
> Cr-Commit-Position: refs/heads/master@{#351465}

TBR=calamity@chromium.org,benwells@chromium.org,jdduke@chromium.org,dominickn@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=464234
Committed: https://crrev.com/4e4ecf3842360f00b08c34a08ff72c9ab20a137f
Cr-Commit-Position: refs/heads/master@{#351511}

[phoglund_chromium, 2015-09-30 07:43:31]

Created Revert of Implement a site engagement score based on time-on-site.

[commit-bot: I haz the power, 2015-09-30 07:44:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1376143003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1376143003/1

[commit-bot: I haz the power, 2015-09-30 07:45:31]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-30 07:46:15]

Patchset 1 (id:??) landed as
https://crrev.com/4e4ecf3842360f00b08c34a08ff72c9ab20a137f
Cr-Commit-Position: refs/heads/master@{#351511}