Refactor class to represent structure.

We had a TTypeLine to represent a structure field,
which simply encapsulated a TType and line number.
The line number was only used during parsing for error reporting.
There is no need to store a line number because it is already
available in the parser token.

TEST=WebGL conformance tests
R=kbr@chromium.org

Review URL: https://codereview.appspot.com/9223045

git-svn-id: http://angleproject.googlecode.com/svn/trunk@2198 736b8ea6-26fd-11df-bfd4-992fa37f6226