Add basic or stub implementations of upstream instance/adapter APIs

Adds upstream instance/adapter APIs. In dawn_native, the basic APIs
to get limits and properties are implemented, but requestAdapter and
requestDevice are not. In dawn_wire, nothing is implemented, but the
stub definitions are put in place, as well the mechanism to inject
WGPUInstance into the wire.

There is a lifetime concern with WGPUInstance and WGPUAdapter on the
wire in that we need to ensure that the client cannot free the
instance or adapter while they are in use. In the near term, this is
not a problem because Chromium will always hold ownership of the
instance and adapters outside of the wire - i.e. it won't inject and
then release ownership.

Bug: dawn:160, dawn:689
Change-Id: Id904272983f23babc9177bc163d78c4fa1044da0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/71520
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

generator/templates/api_cpp.h

@@ -140,7 +140,7 @@ namespace {{metadata.namespace}} {
     };
 
 {% macro render_cpp_default_value(member, is_struct=True) -%}
-    {%- if member.annotation in ["*", "const*"] and member.optional -%}
+    {%- if member.annotation in ["*", "const*"] and member.optional or member.default_value == "nullptr" -%}
         {{" "}}= nullptr
     {%- elif member.type.category == "object" and member.optional and is_struct -%}
         {{" "}}= nullptr

generator/templates/dawn_native/wgpu_structs.h

@@ -21,7 +21,7 @@
 namespace dawn_native {
 
 {% macro render_cpp_default_value(member) -%}
-    {%- if member.annotation in ["*", "const*"] and member.optional -%}
+    {%- if member.annotation in ["*", "const*"] and member.optional or member.default_value == "nullptr" -%}
         {{" "}}= nullptr
     {%- elif member.type.category == "object" and member.optional -%}
         {{" "}}= nullptr