Added pinch viewport offset to page state.

Added pinch viewport offset to page state.

Added field allows Blink to properly restore pinch-zoomed page
when using new "virtual viewport" mode of pinch.

Dependent Blink patch: https://codereview.chromium.org/263853008

BUG=364112
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269724

[aelias_OOO_until_Jul13, 2014-05-02 07:32:50]

Looks mostly OK.  The only problem is we should have a plan for making the
offsets correct when upgrading to the new pinch zoom model and reading an older
historyitem.  Maybe we should write (-1, -1) to the pinch_viewport_scroll_offset
values if --enable-pinch-virtual-viewport is disabled, and then if either
version < 20 or (-1, -1) is seen, distribute the scroll offset

[bokan, 2014-05-06 20:12:55]

Alexandre, PTAL. I've added (-1, -1) as a signal to distribute the scroll
offset. I've also added viewport specific tests for history saving and
restoring.

[aelias_OOO_until_Jul13, 2014-05-07 08:10:19]

lgtm

[bokan, 2014-05-07 12:52:24]

+darin@ for OWNER

[bokan, 2014-05-08 19:42:05]

On 2014/05/07 12:52:24, bokan wrote:
> +darin@ for OWNER

Hi Darin, could you please take a look? Thanks.

[Avi (use Gerrit), 2014-05-09 15:06:39]

lgtm

Looks reasonable.

[bokan, 2014-05-11 16:28:30]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2014-05-11 16:28:42]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/bokan@chromium.org/266673014/20001

[bokan, 2014-05-11 17:40:55]

The CQ bit was unchecked by bokan@chromium.org

[bokan, 2014-05-11 18:00:38]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2014-05-11 18:00:42]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/bokan@chromium.org/266673014/40001

[commit-bot: I haz the power, 2014-05-11 23:23:40]

Change committed as 269724