Disable DownloadDBForNewDownloads when running WebViewTest.DownloadCookieIsolation

This test will work across browser sessions using the history DB.
However, when DownloadDBForNewDownloads is enabled, history DB only stores
download in terminal state.
As a result, relying on history DB doesn't work for this test when
the feature is enabled.
Disable the feature when running the test solve the issue. Will need
to add new test later to test resumption  when the feature is turned on.

BUG=842245

Change-Id: I6c51fc1e2a04a69afe19bbb40cc8fa1b19815732
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536466
Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#643960}