If an extension is not specified, it is disabled by default, thus a shader should fail compiling if features from that extension are used.

ANGLEBUG=204
TEST=webgl conformance/extensions/oes-standard-derivatives.html
Review URL: http://codereview.appspot.com/4974071

git-svn-id: https://angleproject.googlecode.com/svn/trunk@745 736b8ea6-26fd-11df-bfd4-992fa37f6226