Set parent for window picker dialog on Mac

Initially DesktopMediaPicker::Show() didn't have parent parameter, 
so DesktopMediaPickerCococa wasn't setting parent for the window
it creates.

BUG=134249

Review URL: https://codereview.chromium.org/120253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242244 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242244