Change img@currentSrc to be available when image dimensions are known

Following a discussion on https://github.com/ResponsiveImagesCG/picture-element/issues/155 , the currentSrc attribute should be exposed at the same time as the width/height attributes.

BUG=385589

Review URL: https://codereview.chromium.org/331133006

git-svn-id: svn://svn.chromium.org/blink/trunk@176549 bbb929c8-8fbe-4397-9dbb-9b2b20218538