chromeos: Add pref for automated brightness increases.

chromeos: Add pref for automated brightness increases.

Add a pref that controls whether the Chrome OS power manager
will ensure that the screen brightness is nonzero when it
observes user activity.

Also bump the system_api dependency to 7678688b.

BUG=chromium:256914
Committed: https://crrev.com/8f279a0d45ab24506dce54f14a644eeb40f51874
Cr-Commit-Position: refs/heads/master@{#310786}

[Daniel Erat, 2015-01-08 20:41:30]

derat@chromium.org changed reviewers:
+ bartfab@chromium.org, bruthig@chromium.org

[bruthig, 2015-01-08 20:47:49]

On 2015/01/08 20:41:31, Daniel Erat wrote:

lgtm

[bartfab (slow), 2015-01-09 14:57:17]

lgtm

[Daniel Erat, 2015-01-09 15:15:04]

The CQ bit was checked by derat@chromium.org

[commit-bot: I haz the power, 2015-01-09 15:16:13]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/807093003/1

[commit-bot: I haz the power, 2015-01-09 17:12:03]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-09 17:12:56]

Patchset 1 (id:??) landed as
https://crrev.com/8f279a0d45ab24506dce54f14a644eeb40f51874
Cr-Commit-Position: refs/heads/master@{#310786}