When building context menu params, we need a field to indicate whether a image is NULL image.
See method ImageResource::imageForRenderer, it may return a nullImage.
Also add a new field WebContextMenuData::hasImageContents to replace the WebContextMenuData:isImageBlocked later.

BUG=272076
TEST=None
R=jamesr@chromium.org, pkasting@chromium.org

Review URL: https://codereview.chromium.org/22938006

git-svn-id: svn://svn.chromium.org/blink/trunk@156251 bbb929c8-8fbe-4397-9dbb-9b2b20218538