Enable tab discarding on Mac (behind a flag).

Enable tab discarding on Mac (behind a flag).

This enables tab discarding for Mac, hidden behind a command line flag (enable-tab-discarding).

Linux to follow.

Notes:
- This doesn't invoke RecordMemoryStats yet as I'm working towards unifying GetSystemMemoryInfo across all desktop.

- This is CL #9 towards expanding Chromeos tab discarding to other platforms. This enables it for Mac.

BUG=463597
Committed: https://crrev.com/bf286e50cbd2f286fc33e9ece620405ffd17d07c
Cr-Commit-Position: refs/heads/master@{#340266}

[Georges Khalil, 2015-07-23 20:19:29]

Patchset #1 (id:1) has been deleted

[Georges Khalil, 2015-07-23 20:26:11]

georgesak@chromium.org changed reviewers:
+ sky@chromium.org

[Georges Khalil, 2015-07-23 20:26:11]

PTAL.

[sky, 2015-07-23 22:58:31]

Is there a reason you don't want to enable on linux too?

[sky, 2015-07-23 22:59:18]

I'm guessing the underlying measuring isn't that for linux. LGTM

[Georges Khalil, 2015-07-24 02:18:47]

On 2015/07/23 22:59:18, sky wrote:
> I'm guessing the underlying measuring isn't that for linux. LGTM

We need to have memory pressure signals first. We're working on it, then we'll
activate tab discarding.

[Georges Khalil, 2015-07-24 02:18:50]

The CQ bit was checked by georgesak@chromium.org

[commit-bot: I haz the power, 2015-07-24 03:55:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1244033005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1244033005/20001

[commit-bot: I haz the power, 2015-07-24 04:21:18]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-24 04:21:19]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[Georges Khalil, 2015-07-24 12:54:11]

The CQ bit was checked by georgesak@chromium.org

[commit-bot: I haz the power, 2015-07-24 12:54:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1244033005/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1244033005/20001

[commit-bot: I haz the power, 2015-07-24 12:59:55]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-07-24 13:00:33]

Patchset 1 (id:??) landed as
https://crrev.com/bf286e50cbd2f286fc33e9ece620405ffd17d07c
Cr-Commit-Position: refs/heads/master@{#340266}