Reuse Skia functions for converting to/from NSImage.

This also fixes one of the colorspace conversion issues in the Skia Mac
utils tests.

BUG=79946

Review URL: https://codereview.chromium.org/77823003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236223 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236223