Merge RenderWidget into single subclass, RenderPart

The RenderWidget is an old abstraction that isn't needed any longer. We are
down to a single subclass, RenderPart. This CL folds RenderWidget into
RenderPart and updates its users.

BUG=416436

Review URL: https://codereview.chromium.org/625073002

git-svn-id: svn://svn.chromium.org/blink/trunk@183568 bbb929c8-8fbe-4397-9dbb-9b2b20218538