AX: Remove WebLocalFrame dependency from BlinkAXTreeSource.

AX: Remove WebLocalFrame dependency from BlinkAXTreeSource.

This is a preparation to support AXObject in PagePopup.
PagePopup doesn't have WebLocalFrame, and it's difficult to provide it. We can
remove WebLocalFrame dependency of BlinkAXTreeSource by using
WebDocument::scrollOffset, minimumScrollOffset, and maximumScrollOffset.

This CL has no functionality changes.

BUG=123896
Committed: https://crrev.com/e0ce97e9f1afa1f28c1c1261e5372d03b2af1cdb
Cr-Commit-Position: refs/heads/master@{#292839}

[tkent, 2014-08-27 20:36:28]

tkent@chromium.org changed reviewers:
+ dmazzoni@chromium.org

[tkent, 2014-08-27 20:36:28]

dmazzoni@, would you review this please?

[tkent, 2014-09-01 03:14:00]

ping dmazzoni@

[dmazzoni, 2014-09-01 06:04:50]

The CQ bit was checked by dmazzoni@chromium.org

[dmazzoni, 2014-09-01 06:04:51]

lgtm

[commit-bot: I haz the power, 2014-09-01 06:05:27]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/tkent@chromium.org/510123003/1

[commit-bot: I haz the power, 2014-09-01 06:40:03]

Committed patchset #1 (id:1) as 8509614288db831dae8ebed7b4b429d83feed51e

[commit-bot: I haz the power, 2014-09-10 03:15:46]

Patchset 1 (id:??) landed as
https://crrev.com/e0ce97e9f1afa1f28c1c1261e5372d03b2af1cdb
Cr-Commit-Position: refs/heads/master@{#292839}