Reland "Re-enable testRepostFormDismissedByNewNavigation"

This is a reland of ca98599d98a8cfcce42dd17065b9c42610185af5

The first attempt at re-enabling this test did not work for iPads. The
disabling of synchronization must be limited to only the wait for the resent post
alert.

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}

Bug: 971670
Change-Id: I6e0a28739e5ec8d57d036c397a4943c96b178576
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1683670
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#674152}
1 file changed