Make sure content offset gets updated

Make sure content offset gets updated

On Webapp, touch gestures are offset by the size of the toolbar, even when
the toolbar is hidden. This occurs because we assume that the previous
content offset can never be 0, ending up not calling
onVisibleContentOffsetChanged() callback.

We can fix it by initializing previous content offset as NaN.

BUG=459275
Committed: https://crrev.com/19f04a05b39e5586cba79eaba009ae675ca6aad1
Cr-Commit-Position: refs/heads/master@{#318974}

[Changwan Ryu, 2015-03-03 05:22:19]

changwan@chromium.org changed reviewers:
+ dfalcantara@chromium.org, dtrainor@chromium.org, tedchoc@chromium.org

[David Trainor- moved to gerrit, 2015-03-03 06:28:28]

+1 but let Ted confirm.

[Ted C, 2015-03-03 19:13:36]

lgtm

[Changwan Ryu, 2015-03-03 23:48:09]

The CQ bit was checked by changwan@chromium.org

[commit-bot: I haz the power, 2015-03-03 23:49:27]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/970813003/1

[commit-bot: I haz the power, 2015-03-04 00:16:23]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-04 00:17:33]

Patchset 1 (id:??) landed as
https://crrev.com/19f04a05b39e5586cba79eaba009ae675ca6aad1
Cr-Commit-Position: refs/heads/master@{#318974}