Improve validation of Gen/Delete calls

Add checks for negative count to GenTransformFeedbacks and
DeleteTransformFeedbacks, and check for active transform feedbacks in
DeleteTransformFeedbacks.

Unify validation and error messages of all other Gen/Delete calls.

BUG=angleproject:1101
TEST=dEQP-GLES3.functional.negative_api.* (two more tests pass)

Change-Id: I128063fab3db27a25e282a10c916c53646d68b9c
Reviewed-on: https://chromium-review.googlesource.com/332142
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index ebda16a..ee5ac41 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -1050,7 +1050,7 @@
         return false;
     }
 
-    return ValidateDeleteVertexArraysBase(context, n);
+    return ValidateGenOrDelete(context, n);
 }
 
 bool ValidateGenVertexArraysOES(Context *context, GLsizei n)
@@ -1061,7 +1061,7 @@
         return false;
     }
 
-    return ValidateGenVertexArraysBase(context, n);
+    return ValidateGenOrDelete(context, n);
 }
 
 bool ValidateIsVertexArrayOES(Context *context)