More shader refactor cleanup

Removed VertexShader and FragmentShader in favor of using a single Shader object
Used VertexShaderD3D and FragmentShaderD3D directly when appropriate.

BUG=angle:731
Change-Id: Ib1ffdf85ee0bfa7c88bb9ddb1029ccbd63a9dbe1
Reviewed-on: https://chromium-review.googlesource.com/213840
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Brandon Jones <bajones@chromium.org>
diff --git a/src/libGLESv2/Program.cpp b/src/libGLESv2/Program.cpp
index 6e38412..9236d22 100644
--- a/src/libGLESv2/Program.cpp
+++ b/src/libGLESv2/Program.cpp
@@ -173,7 +173,7 @@
             return false;
         }
 
-        mVertexShader = (VertexShader*)shader;
+        mVertexShader = shader;
         mVertexShader->addRef();
     }
     else if (shader->getType() == GL_FRAGMENT_SHADER)
@@ -183,7 +183,7 @@
             return false;
         }
 
-        mFragmentShader = (FragmentShader*)shader;
+        mFragmentShader = shader;
         mFragmentShader->addRef();
     }
     else UNREACHABLE();