Stop injecting an ExternalDataFetcher in ExternalData browser test

The DeviceLocalAccountExternalDataTest.ExternalData test had to inject
an ExternalDataFetcher while no policy referencing external data existed.
Now that the first such policy has been added, this CL switches the test
to it, avoiding the need to inject a fetcher.

BUG=None
TEST=Updated browser test

Review URL: https://codereview.chromium.org/95403005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238089 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238089