Don't try to set transparency on pixbufs that don't have any transparency channel.
Don't try to set transparency on pixbufs that don't have any transparency channel.

Also add DCHECK to make sure the assumption rowstride >= width * 4 is true.

The reason this didn't cause any crashes or graphical corruption is because the non-transparent images don't have any white in them (luckily we are not making a browser for an ipod).

BUG=16749
TEST=valgrind the browser, open find bar

Review URL: http://codereview.chromium.org/155547

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@20701 0039d316-1c4b-4281-b951-d872f2087c98

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