Update BuildFakeGaiaCookieManagerService to no longer create a TestUrlLoaderFactory by default.

Once FakeGaiaCookieManagerService is eliminated, tests will need to own the TestUrlLoaderFactory
themselves, so it's better not to have a TestUrlLoaderFactory be created except when it will
actually be used.

For tests which actually need a TestUrlLoaderFactory we now set
create_fake_url_loader_factory_for_cookie_requests explicitly to true.

This is part of step 1a of the plan to eliminate FakeGCMS entirely:
https://docs.google.com/document/d/1t0ZtuV7h-znzdItFgBW0aKPscAwWXIBuNZnNlEGgi7g/edit

TBR=menegola@chromium.org,mamir@chromium.org,avi@chromium.org,bsep@chromium.org

Bug: 907782
Change-Id: I23e7dcb176f384ebd6b165a489a427be54a47399
Reviewed-on: https://chromium-review.googlesource.com/c/1378091
Commit-Queue: Lowell Manners <lowell@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617111}