Element: Use $.context instead of recomputing in #getElementDocument

In the constructor we already call getJQuery, which uses the
same static.getElementDocument and stores it in wrapper.context.

The constructor even uses this (this.$.context.createElement).

Simplfy the non-static getElementDocument by simply returning that
instead of recomputing it.

Change-Id: I678cbe21ac3dec02498428e727a51ea42c4cb74b