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This CL adds a new LiveSigninTest::CancelSyncWithWebAccount to the suite
of live signin tests.

The test does the following:
- Signs in on the web
- Goes to the settings page and starts the enable Sync flow
- Cancels the sync confirmation dialog
- Checks that the account is still signed in but Sync is disabled

This CL adds a new method to cancel the sync confirmation dialog to
login_ui_test_utils.

The CL also fixes a bug in the javascript injected to the sync
confirmation dialog from
SigninViewControllerTestUtil::TryDismissSyncConfirmationDialog.
"document.querySelector('sync-confirmation-app')" may evaluate in null
that causes the subsequent attempt to access "shadowRoot" to throw an
exception. content::ExecuteScriptAndExtractString(), however, stays
blocked because it waits for a message to be sent through
"window.domAutomationController.send()". All of this makes the test
timeout.

Bug: 1008891
Change-Id: I6793e40d400ca62f97ea1d60b99b628aabefb369
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1944389Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720444}