Add varying packing validation for WebGL.

This CL moves the varying packing from the D3D layer up to Program.
This is necessary for WebGL validation, and gives us consistency
for the various back-ends. There may be some additional cleanup work
on the VaryingPacking class, because it does some work that is D3D-
specific.

WebGL requires strict varying packing. Instead of allowing success
unconditionally, it's an explicit error to succeed to pack a set of
varyings that the sample algorithm would fail to pack.

Introduce a new packing mode option to the varying packing class to
handle this different packing style, while keeping our old more
relaxed packing method for ES code.

BUG=angleproject:1675

Change-Id: I674ae685ba573cc2ad7d9dfb7441efa8cb2d55fc
Reviewed-on: https://chromium-review.googlesource.com/423254
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/renderer/gl/ProgramGL.cpp b/src/libANGLE/renderer/gl/ProgramGL.cpp
index d04c1e7..06e7233 100644
--- a/src/libANGLE/renderer/gl/ProgramGL.cpp
+++ b/src/libANGLE/renderer/gl/ProgramGL.cpp
@@ -111,7 +111,9 @@
     }
 }
 
-LinkResult ProgramGL::link(const gl::ContextState &data, gl::InfoLog &infoLog)
+LinkResult ProgramGL::link(const gl::ContextState &data,
+                           const gl::VaryingPacking &packing,
+                           gl::InfoLog &infoLog)
 {
     preLink();