Encode browser action icons as PNG when saving them to disk. This fixes a
DCHECK which triggered when trying to interpret raw SkBitmap data.

BUG=326374

Review URL: https://codereview.chromium.org/113583002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240280 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240280