Make TestingProfile auto-create incognito profiles.

Previously, tests that involved incognito profiles needed to manually
create them and add them to the parent profile before
GetOffTheRecordProfile was ever called.  However, the documentation for
Profile explicitly states that the incognito profile will be created if
it doesn't already exist when GetOffTheRecordProfile is called. Further,
it was prohibitvely invasive to test code that creates a profile and
expects to be able to do something with its associated incognito
profile.

This commit also updates existing tests to remove unnecessary manual
incognito profile creation.

BUG=318803
R=atwilson@chromium.org, dpolukhin@chromium.org, sky@chromium.org

Review URL: https://codereview.chromium.org/181403006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255647 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255647