Refactor WebCore::DragImage.

This CL combines DragImage and DragImageChromiumSkia and changes style
to be more object-oriented and use smart pointers (to clarify ownership
semantics). This makes the resulting code smaller and more consistent
with the rest of Blink.

BUG=

Review URL: https://chromiumcodereview.appspot.com/16715002

git-svn-id: svn://svn.chromium.org/blink/trunk@152692 bbb929c8-8fbe-4397-9dbb-9b2b20218538
17 files changed