Make PackedVarying a D3D-only type.

The register assignment stuff only applies to the D3D back-end.
Cleans up the GL back-ends use of PackedVarying, and will lead
to future cleanups relating to packing varyings.

BUG=angleproject:1123

Change-Id: Iaaa5fc03577e5b61ea6ae76ee1e15ad608037f34
Reviewed-on: https://chromium-review.googlesource.com/295190
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/Program.cpp b/src/libANGLE/Program.cpp
index 271502d..ca58853 100644
--- a/src/libANGLE/Program.cpp
+++ b/src/libANGLE/Program.cpp
@@ -1204,10 +1204,10 @@
                            const Shader *vertexShader,
                            const Shader *fragmentShader)
 {
-    const std::vector<PackedVarying> &vertexVaryings   = vertexShader->getVaryings();
-    const std::vector<PackedVarying> &fragmentVaryings = fragmentShader->getVaryings();
+    const std::vector<sh::Varying> &vertexVaryings   = vertexShader->getVaryings();
+    const std::vector<sh::Varying> &fragmentVaryings = fragmentShader->getVaryings();
 
-    for (const PackedVarying &output : fragmentVaryings)
+    for (const sh::Varying &output : fragmentVaryings)
     {
         bool matched = false;
 
@@ -1217,7 +1217,7 @@
             continue;
         }
 
-        for (const PackedVarying &input : vertexVaryings)
+        for (const sh::Varying &input : vertexVaryings)
         {
             if (output.name == input.name)
             {