Move GrShaderVar to an appending model for layout qualifiers

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5144

Change-Id: I05e88961e50bd679cfe5863f413f3ec65bc9fd95
Reviewed-on: https://skia-review.googlesource.com/5144
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/src/gpu/vk/GrVkVaryingHandler.cpp b/src/gpu/vk/GrVkVaryingHandler.cpp
index 2b3e975..9bab292 100644
--- a/src/gpu/vk/GrVkVaryingHandler.cpp
+++ b/src/gpu/vk/GrVkVaryingHandler.cpp
@@ -58,7 +58,7 @@
         GrShaderVar& var = vars[i];
         SkString location;
         location.appendf("location = %d", locationIndex);
-        var.setLayoutQualifier(location.c_str());
+        var.addLayoutQualifier(location.c_str());
 
         int elementSize = grsltype_to_location_size(var.getType());
         SkASSERT(elementSize);