Link atomic counters to buffers

Gather counters from each shader and group them according the
layout qualifier 'binding' into each buffer.

BUG=angleproject:1729
TEST=angle_end2end_tests:AtomicCounterBufferTest

Change-Id: I8d0cd0d2bf65be37c035b0e1540481c8bee0bae4
diff --git a/src/libANGLE/validationES.cpp b/src/libANGLE/validationES.cpp
index 3d4fa1f..96f13c9 100644
--- a/src/libANGLE/validationES.cpp
+++ b/src/libANGLE/validationES.cpp
@@ -518,7 +518,7 @@
         {
             const UniformBlock &uniformBlock =
                 programObject->getUniformBlockByIndex(uniformBlockIndex);
-            *length = static_cast<GLsizei>(uniformBlock.memberUniformIndexes.size());
+            *length = static_cast<GLsizei>(uniformBlock.memberIndexes.size());
         }
         else
         {