Add an option to support for GL_OES_EGL_image_external.

Comes with this extension is the new sampler type samplerExternalOES.

ANGLEBUG=175
TEST=compile the attached shader file
Review URL: http://codereview.appspot.com/4809076

git-svn-id: https://angleproject.googlecode.com/svn/trunk@728 736b8ea6-26fd-11df-bfd4-992fa37f6226
diff --git a/src/compiler/TranslatorESSL.cpp b/src/compiler/TranslatorESSL.cpp
index bb201a4..794236b 100644
--- a/src/compiler/TranslatorESSL.cpp
+++ b/src/compiler/TranslatorESSL.cpp
@@ -28,7 +28,9 @@
     const TExtensionBehavior& extensionBehavior = getExtensionBehavior();
     for (TExtensionBehavior::const_iterator iter = extensionBehavior.begin();
          iter != extensionBehavior.end(); ++iter) {
-        sink << "#extension " << iter->first << " : "
-             << getBehaviorString(iter->second) << "\n";
+        if (iter->second != EBhUndefined) {
+            sink << "#extension " << iter->first << " : "
+                 << getBehaviorString(iter->second) << "\n";
+        }
     }
 }