Revert of Remove a HistoryNode's children when it navigates to a new document.

Revert of Remove a HistoryNode's children when it navigates to a new document. (patchset #1 id:1 of https://codereview.chromium.org/1139033002/)

Reason for revert:
Caused a large spike in crashes across several bugs:
https://crbug.com/426658
https://crbug.com/488382
https://crbug.com/488421
https://crbug.com/488434
https://crbug.com/488470

Original issue's description:
> Remove a HistoryNode's children when it navigates to a new document.
>
> When a reload occurs (or other inert commits), we leave the history tree alone. However, if iframes are dynamically created, we end up with both the old iframes and the new iframes in the history tree, and we never prune them. Instead, if a different-document inert commit occurs, remove the committing node's children, since the corresponding frames are no longer in the frame tree.
>
> BUG=485385
>
> Committed: https://crrev.com/9f3e29c910ce2c2a9c3a60b2675db59d95b8ad3a
> Cr-Commit-Position: refs/heads/master@{#329763}

TBR=japhet@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=485385
Committed: https://crrev.com/5c8a902ebf5277d4c621d9c190cf659fbebcfc7a
Cr-Commit-Position: refs/heads/master@{#330435}

[Charlie Reis, 2015-05-18 21:46:40]

Created Revert of Remove a HistoryNode's children when it navigates to a new
document.

[commit-bot: I haz the power, 2015-05-18 21:47:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1148473002/1

[commit-bot: I haz the power, 2015-05-18 21:49:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-18 22:44:22]

Patchset 1 (id:??) landed as
https://crrev.com/5c8a902ebf5277d4c621d9c190cf659fbebcfc7a
Cr-Commit-Position: refs/heads/master@{#330435}