remove PNGCodec::CreateSkBitmapFromBGRAFormat -- not referenced

optimizations I was attempting.. until I discovered it is never called.
1. need to set opaque to true when we can (it never does in the current code)
2. the scale by alpha is numerically off by 1 for opaque-alpha

BUG=302003

Review URL: https://codereview.chromium.org/25251004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226117 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226117