Use null rather than empty frame to lock Surface resources

Use null rather than empty frame to lock Surface resources

This will allow the DisplayScheduler to differentiate between
a frame without any resources and one that should not be drawn.

Some tests use frames without any resources and the DisplayScheduler
was missinterpreting that as the Browser locking resources.

BUG=467750, 476544
Committed: https://crrev.com/9576564d395425ce1c073fa3f33b8d5c062add8e
Cr-Commit-Position: refs/heads/master@{#325369}

[brianderson, 2015-04-15 23:29:31]

brianderson@chromium.org changed reviewers:
+ jbauman@chromium.org

[brianderson, 2015-04-15 23:29:32]

ptal

[jbauman, 2015-04-15 23:32:17]

lgtm

[brianderson, 2015-04-15 23:33:30]

The CQ bit was checked by brianderson@chromium.org

[brianderson, 2015-04-15 23:33:34]

The CQ bit was unchecked by brianderson@chromium.org

[brianderson, 2015-04-15 23:34:30]

The CQ bit was checked by brianderson@chromium.org

[commit-bot: I haz the power, 2015-04-15 23:35:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1084313002/1

[commit-bot: I haz the power, 2015-04-16 01:33:01]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-16 01:33:53]

Patchset 1 (id:??) landed as
https://crrev.com/9576564d395425ce1c073fa3f33b8d5c062add8e
Cr-Commit-Position: refs/heads/master@{#325369}