Revert "Add about:flag for feature PdfIsolation"

This reverts commit 3f7270c272c98bafff4c7943e56d9884ae41ff86.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Add about:flag for feature PdfIsolation
> 
> Bug: 809614
> Change-Id: I63fb89898c9ed83e532fd148a0a4dd318c194693
> Reviewed-on: https://chromium-review.googlesource.com/956488
> Commit-Queue: Tom Sepez <tsepez@chromium.org>
> Reviewed-by: John Abd-El-Malek <jam@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542215}

TBR=isherman@chromium.org,jam@chromium.org,tsepez@chromium.org

Change-Id: Ifec7ccfc80d125bdef35f41f997aa412db7a8823
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 809614
Reviewed-on: https://chromium-review.googlesource.com/957784
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542287}
4 files changed