Use FakeDesktopMediaList for Desktop Capture API tests.

Previously the tests were using actual media sources. This change makes
the test more reliable as they no longer depend on system resources.
Also FakeDesktopMediaList has been moved from
desktop_media_picker_controller_unittest.mm to a separate file so it can
be reused

R=mpcomplete@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/100303004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238463 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238463