Remove last use of WebFrame::isLoading()

Remove last use of WebFrame::isLoading()

All other uses are from a WebLocalFrame, and the concept is local-specific.
This will enable us to move it to WebLocalFrame in the blink repo.

BUG=

Committed: https://crrev.com/02cfd5ccc819ce649db8ff032d43fb16ee3167d9
Cr-Commit-Position: refs/heads/master@{#293869}

[Nate Chapin, 2014-09-08 20:00:57]

japhet@chromium.org changed reviewers:
+ vitalybuka@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-09-08 23:20:47]

The CQ bit was checked by vitalybuka@chromium.org

[Vitaly Buka (NO REVIEWS), 2014-09-08 23:20:47]

lgtm

[commit-bot: I haz the power, 2014-09-08 23:29:55]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/japhet@chromium.org/557483002/1

[commit-bot: I haz the power, 2014-09-09 06:21:36]

Committed patchset #1 (id:1) as 11a9c50dc0826a48e83b6d22ffbcde0333ba7d57

[commit-bot: I haz the power, 2014-09-10 03:51:07]

Patchset 1 (id:??) landed as
https://crrev.com/02cfd5ccc819ce649db8ff032d43fb16ee3167d9
Cr-Commit-Position: refs/heads/master@{#293869}