Implement support of unary operator "+" in translator.

BUG=angle:779
TEST=conformance/glsl/misc/struct-unary-operators.html

Change-Id: Ia827e07dcfc8ad3bbbc078e54336815be9027945
Reviewed-on: https://chromium-review.googlesource.com/222720
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>
diff --git a/src/compiler/translator/ValidateLimitations.cpp b/src/compiler/translator/ValidateLimitations.cpp
index c1a7b75..896e1cd 100644
--- a/src/compiler/translator/ValidateLimitations.cpp
+++ b/src/compiler/translator/ValidateLimitations.cpp
@@ -94,6 +94,7 @@
       case EOpLogicalXor: return "^^";
       case EOpLogicalAnd: return "&&";
       case EOpNegative: return "-";
+      case EOpPositive: return "+";
       case EOpVectorLogicalNot: return "not";
       case EOpLogicalNot: return "!";
       case EOpPostIncrement: return "++";