Follow up RenderPart refactoring

* Remove unnecessary RenderPart.h imports
* s/updateAfterWidgetResize/updateAfterPartResize/

BUG=416436

Review URL: https://codereview.chromium.org/652293002

git-svn-id: svn://svn.chromium.org/blink/trunk@183680 bbb929c8-8fbe-4397-9dbb-9b2b20218538