Change WebContents::last_active_time_ to Time instead of Timeticks.

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}

[Georges Khalil, 2015-05-13 19:58:09]

Patchset #1 (id:1) has been deleted

[Georges Khalil, 2015-05-13 20:43:58]

Patchset #1 (id:20001) has been deleted

[Georges Khalil, 2015-05-13 20:44:49]

Patchset #1 (id:40001) has been deleted

[Georges Khalil, 2015-05-13 20:45:04]

Patchset #1 (id:60001) has been deleted

[Georges Khalil, 2015-05-14 14:57:00]

Patchset #1 (id:80001) has been deleted

[Georges Khalil, 2015-05-14 15:15:47]

Patchset #1 (id:100001) has been deleted

[Georges Khalil, 2015-05-14 15:17:56]

Patchset #1 (id:120001) has been deleted

[Georges Khalil, 2015-05-14 16:43:44]

georgesak@chromium.org changed reviewers:
+ jam@chromium.org, jamescook@chromium.org

[Georges Khalil, 2015-05-14 16:43:45]

jamescook@chromium.org: Please review changes in 
chrome/browser/chromeos/memory/*

jam@chromium.org: Please review changes in rest.

[James Cook, 2015-05-14 21:50:57]

lgtm

[jam, 2015-05-14 22:22:26]

lgtm

[Georges Khalil, 2015-05-15 02:56:42]

The CQ bit was checked by georgesak@chromium.org

[commit-bot: I haz the power, 2015-05-15 02:57:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1140083004/140001

[commit-bot: I haz the power, 2015-05-15 03:01:59]

Committed patchset #1 (id:140001)

[commit-bot: I haz the power, 2015-05-15 03:02:52]

Patchset 1 (id:??) landed as
https://crrev.com/041ef9c96d9a5b6f206a24385b0e6e4b3dbf9f20
Cr-Commit-Position: refs/heads/master@{#330029}

[gab, 2015-05-20 18:41:51]

A revert of this CL (patchset #1 id:140001) has been created in
https://codereview.chromium.org/1145233002/ by gab@chromium.org.

The reason for reverting is: Reverting for georgesak@ (his committer access
hasn't propagated yet it looks like).

After discussion with sky@, decided to keep TimeTicks..