canvas.getContext with alpha:undefined should behave like alpha:true

WebIDL dictionary spec says that setting a dictionary value to null or
undefined should result in the attribute retaining its default value.
So, if alpha is undefined or null, it should be converted as true.

BUG=371089

Review URL: https://codereview.chromium.org/290143007

git-svn-id: svn://svn.chromium.org/blink/trunk@174508 bbb929c8-8fbe-4397-9dbb-9b2b20218538