Speed up the PolicyToPrefsMapping browser test

This CL speeds up the PolicyToPrefsMapping test by an order of magnitude
by executing all test cases in one go, instead of instantiating one test
per pref. The resulting single test is still fast enough that it should
happily pass even on slow bots.

On my Linux machine, the change in wall clock time needed by the tests is:

Release:  7:34 minutes -> 3 seconds
Debug:   11:02 minutes -> 7 seconds

BUG=325205
TEST=Updated browser test

Review URL: https://codereview.chromium.org/101983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238685 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238685