Add a MultiProcessTest::SpawnChildWithOptions().

Replace SpawnChildImpl() with SpawnChildWithOptions(). Leave the
now-superfluous POSIX-only SpawnChild() that takes a
FileHandleMappingVector for now.

R=brettw@chromium.org, brettw

Review URL: https://codereview.chromium.org/104803007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239258 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239258