After the shader compile (and before code output), the precision scopes are already lost.  In order to correctly output precisions, we need to emit precision in each variable declaration, therefore, each variable should have its precision set.  This CL fixes the bugs that the precisions are lost for variables using default precsions and struct fields.  Also, this CL fixes a bug in the grammar: constructors are not type_specifier and they shouldn't have precisions.

BUG=168
TEST=webgl conformance tests, gles2 conformance tests.
Review URL: http://codereview.appspot.com/4617041

git-svn-id: https://angleproject.googlecode.com/svn/trunk@695 736b8ea6-26fd-11df-bfd4-992fa37f6226