Implement External Texture Binding Functionality

Adds functionality to BindGroupLayout and BindGroup to allow
GPUExternalTexture bindings.

Bug: dawn:728
Change-Id: I651b28606dceda15f0a944711ddba639df77c1a3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/47381
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn_native/opengl/PipelineLayoutGL.cpp b/src/dawn_native/opengl/PipelineLayoutGL.cpp
index 088eaf3..ef2cf7c 100644
--- a/src/dawn_native/opengl/PipelineLayoutGL.cpp
+++ b/src/dawn_native/opengl/PipelineLayoutGL.cpp
@@ -58,6 +58,7 @@
                         break;
 
                     case BindingInfoType::Texture:
+                    case BindingInfoType::ExternalTexture:
                         mIndexInfo[group][bindingIndex] = sampledTextureIndex;
                         sampledTextureIndex++;
                         break;