Added previously missing check for embedded structs; even though these
attempts would be caught by an underlying GLSL compiler, the shader
validator should not let them through.

BUG=http://code.google.com/p/angleproject/issues/detail?id=235
TEST=WebGL conformance tests
Review URL: http://codereview.appspot.com/5327046

git-svn-id: https://angleproject.googlecode.com/svn/trunk@809 736b8ea6-26fd-11df-bfd4-992fa37f6226