Gallery: fix coordinate calculation of viewport.js.

Gallery: fix coordinate calculation of viewport.js.

We haven't considered top margin of screen when we calculate the
centered rect before. This caused wrong animations of crop and rotate.

BUG=521516, 521541
TEST=manually tested as described in the issues.

Committed: https://crrev.com/e962293dc947e79041a1a2179e6910dec25d5315
Cr-Commit-Position: refs/heads/master@{#343835}

[yawano, 2015-08-18 05:32:17]

yawano@chromium.org changed reviewers:
+ hirono@chromium.org

[yawano, 2015-08-18 05:32:18]

PTAL. Thank you!

[hirono, 2015-08-18 05:37:58]

lgtm

[yawano, 2015-08-18 05:41:15]

On 2015/08/18 05:37:58, hirono wrote:
> lgtm

Thank you!

[yawano, 2015-08-18 05:41:19]

The CQ bit was checked by yawano@chromium.org

[commit-bot: I haz the power, 2015-08-18 05:41:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1294303003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1294303003/1

[commit-bot: I haz the power, 2015-08-18 06:24:26]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-18 06:25:13]

Patchset 1 (id:??) landed as
https://crrev.com/e962293dc947e79041a1a2179e6910dec25d5315
Cr-Commit-Position: refs/heads/master@{#343835}