chrome/app/theme: Moved Mac- and Win-specific icon files into subdir.

Inside 'chromium', there are now 'mac' and 'win' subdirectories for
platform-specific files.

BUG=333751
R=cpu@chromium.org, oshima@chromium.org

Review URL: https://codereview.chromium.org/136243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244619 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244619