[xtext][highlighting] Need to process initialized resource

Derived state aware resources need to be initialized before
coloring is applied. getContents() needs to be called

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=454219

Change-Id: I54e1c7b01950bc19022d50f03f748aa3d9cb45a9
Merge "[xtext][highlighting] Need to process initialized resource"