A special state to track field selection is not necessary.

Identifier is sufficient to parse struct fieldname.
Note that we do not need stacked lexer state anymore.

TEST=WebGL conformance tests.

Review URL: https://codereview.appspot.com/8565045

git-svn-id: http://angleproject.googlecode.com/svn/trunk@2191 736b8ea6-26fd-11df-bfd4-992fa37f6226