ozone: Directly provide size of active mode from HardwareDisplayController

ozone: Directly provide size of active mode from HardwareDisplayController

This change adds a GetModeSize() method that returns the size of the
HDC's output as a convenience.

TEST=Boot chrome on link_freon (and attach an external display.)
BUG=None

Committed: https://crrev.com/a436b4f19b34bc9ae667530d7cf38916b8237172
Cr-Commit-Position: refs/heads/master@{#293161}

[robert.bradford, 2014-09-03 15:24:08]

robert.bradford@intel.com changed reviewers:
+ dnicoara@chromium.org

[dnicoara, 2014-09-03 15:28:23]

lgtm

Thanks!

[robert.bradford, 2014-09-03 16:10:28]

The CQ bit was checked by robert.bradford@intel.com

[commit-bot: I haz the power, 2014-09-03 16:14:55]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/robert.bradford@intel.com/535963002/1

[commit-bot: I haz the power, 2014-09-03 17:26:09]

Committed patchset #1 (id:1) as da6831befb3aabe6288dd29e881f8949547d7ec5

[commit-bot: I haz the power, 2014-09-10 03:26:33]

Patchset 1 (id:??) landed as
https://crrev.com/a436b4f19b34bc9ae667530d7cf38916b8237172
Cr-Commit-Position: refs/heads/master@{#293161}