Revert "Re-enable testRepostFormDismissedByNewNavigation"

This reverts commit ca98599d98a8cfcce42dd17065b9c42610185af5.

Reason for revert: Turned slimnav red.

Original change's description:
> Re-enable testRepostFormDismissedByNewNavigation
> 
> EarlGrey synchronization needs to be disabled in order to load a new page while
> the confirmation dialog is shown. However it was being disabled before the
> confirmation dialog was presented. This was causing the dialog presented matcher
> to fail prematurely. This failure can be prevented by disabling synchronization
> after the dialog is presented.
> 
> Bug: 971670
> Change-Id: I1181821f31211e9b45933df429a286132eba8c6c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1682875
> Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
> Auto-Submit: Mike Dougherty <michaeldo@chromium.org>
> Reviewed-by: Eugene But <eugenebut@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#673745}

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

Change-Id: I828bbe242f8cdd9e4767d0edcf348852d605eb96
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 971670
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1682726
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673811}