Extract unit tests from the mixed bag process_util_unittest.cc into proper files.

It is a first step of multiple CLs.

Rename process_metrics_unittests.cc to process_metrics_unittest.cc to be
consistent with the rest of the project.

Rename process_util_unittest_ios.cc to process_metrics_unittest_ios.cc since it
is just about process_metrics.h.

Extract a few process_metrics.h specific unit tests from
process_util_unittest.cc into process_metrics_unittest.cc.

R=thakis@chromium.org
BUG=324546

Review URL: https://codereview.chromium.org/78893002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238009 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238009