Implement GL_EXT_texture_norm16 extension

BUG=angleproject:1365

Change-Id: I972b156ecb41ff44bc1f6365373ab386bacc85f1
Reviewed-on: https://chromium-review.googlesource.com/346530
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 72ff2a5..2ef7972 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -2090,6 +2090,7 @@
         // Disable ES3+ extensions
         mExtensions.colorBufferFloat = false;
         mExtensions.eglImageExternalEssl3 = false;
+        mExtensions.textureNorm16         = false;
     }
 
     if (mClientVersion > 2)