Check that the incoming type to a default precision qualifier statement is valid.

Fixes new WebGL conformance tests
shader-with-illegal-default-precision.frag.html and
shader-with-illegal-default-precision.vert.html per
http://www.khronos.org/bugzilla/show_bug.cgi?id=781 . Also ran WebGL
1.0.1 conformance tests; no regressions.

BUG=none
Review URL: https://codereview.appspot.com/7302043

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1824 736b8ea6-26fd-11df-bfd4-992fa37f6226