Collapse ImageSkia.cpp and ImageChromium.cpp into Image.cpp

Also remove declaration of Image::create, which had no implementation in
any of these files (or anywhere else!)

BUG=242060

Review URL: https://chromiumcodereview.appspot.com/21398002

git-svn-id: svn://svn.chromium.org/blink/trunk@155329 bbb929c8-8fbe-4397-9dbb-9b2b20218538