Fix extension behavior in shader validation.

Fix extension behavior in shader validation.

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
Committed: http://code.google.com/p/angleproject/source/detail?r=745

[zmo, 2011-09-09 01:25:23]

Tested manually with the shader in the conformance test.  Please review.

[kbr1, 2011-09-09 01:36:34]

LGTM