Revert "[ios] Extend the life time of navigation context to error placeholder."

This reverts commit 66d735a8758616bcedf19cefce8554d06de16b50.

Reason for revert: Suspected cause for the new failures on ios-webview
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/ios-webview/2465

Original change's description:
> [ios] Extend the life time of navigation context to error placeholder.
> 
> Also deferred WebStateObserver::DidFinishNavigation callback to after
> placeholder load finishes. This removes an inconsistency between legacy
> nav and slim nav handling of errors.
> 
> Bug: 903497
> Change-Id: I7ee20859b3293da305e2a0ba15d017ea7be0ced8
> Reviewed-on: https://chromium-review.googlesource.com/c/1357613
> Commit-Queue: Danyao Wang <danyao@chromium.org>
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#613573}

TBR=eugenebut@chromium.org,danyao@chromium.org

Change-Id: Ib83d75bca88feb7833f1100c297391caaae6d355
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 903497
Reviewed-on: https://chromium-review.googlesource.com/c/1361640
Reviewed-by: Robert Flack <flackr@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613593}
5 files changed