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/OutputGLSLBase.cpp b/src/compiler/translator/OutputGLSLBase.cpp
index 6d07ccc..d03abb8 100644
--- a/src/compiler/translator/OutputGLSLBase.cpp
+++ b/src/compiler/translator/OutputGLSLBase.cpp
@@ -395,6 +395,7 @@
     switch (node->getOp())
     {
       case EOpNegative: preString = "(-"; break;
+      case EOpPositive: preString = "(+"; break;
       case EOpVectorLogicalNot: preString = "not("; break;
       case EOpLogicalNot: preString = "(!"; break;