Revert of Change WebContents::last_active_time_ to Time instead of Timeticks.

Revert of Change WebContents::last_active_time_ to Time instead of Timeticks. (patchset #1 id:140001 of https://codereview.chromium.org/1140083004/)

Reason for revert:
Reverting for georgesak@ (his committer access hasn't propagated yet it looks like).

After discussion with sky@, decided to keep TimeTicks.

Original issue's description:
> Change WebContents::last_active_time_ to Time instead of Timeticks.
>
> For context, last_active_time_ is going to be used by session restore to order the loading of background tabs using MRU. In order for this to be robust, last_active_time_ must be saved and restore between sessions. Timeticks cannot be reliably restored as it's dependent on the current OS session. MRU code for session restore is being implemented in https://codereview.chromium.org/1131373003
>
> Notes:
> - In dev tools, replaced "activity" with "active" for consistency
> - In OomPriorityManagerTest.Comparator, initialized last_active_time for all tabs, as this is necessary with Time (a default TimeTicks can go back in time, not the case with Time).
>
> BUG=472772
>
> Committed: https://crrev.com/041ef9c96d9a5b6f206a24385b0e6e4b3dbf9f20
> Cr-Commit-Position: refs/heads/master@{#330029}

TBR=jamescook@chromium.org,jam@chromium.org,georgesak@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=472772
Committed: https://crrev.com/0dccfef67664a8304772e46696be7f15ee177419
Cr-Commit-Position: refs/heads/master@{#330778}

[gab, 2015-05-20 18:41:52]

Created Revert of Change WebContents::last_active_time_ to Time instead of
Timeticks.

[commit-bot: I haz the power, 2015-05-20 18:42:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1145233002/1

[commit-bot: I haz the power, 2015-05-20 18:43:53]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-20 18:44:52]

Patchset 1 (id:??) landed as
https://crrev.com/0dccfef67664a8304772e46696be7f15ee177419
Cr-Commit-Position: refs/heads/master@{#330778}