Implement gl_VertexID

BUG=angleproject:1217

Change-Id: Ibb9423d7de4966bce231734925a804b6340b5059
Reviewed-on: https://chromium-review.googlesource.com/321420
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/translator/Initialize.cpp b/src/compiler/translator/Initialize.cpp
index 2f51aad..410d6df 100644
--- a/src/compiler/translator/Initialize.cpp
+++ b/src/compiler/translator/Initialize.cpp
@@ -578,6 +578,8 @@
             TType(EbtFloat, EbpMedium, EvqPointSize, 1)));
         symbolTable.insert(ESSL3_BUILTINS, new TVariable(NewPoolTString("gl_InstanceID"),
             TType(EbtInt, EbpHigh, EvqInstanceID, 1)));
+        symbolTable.insert(ESSL3_BUILTINS, new TVariable(NewPoolTString("gl_VertexID"),
+                                                         TType(EbtInt, EbpHigh, EvqVertexID, 1)));
         break;
 
       default: