Do not memcpy transparent wire structs on serialization

This removes a serialization optimization where structs could be
memcpy'ed. memcpy is still used for deserialization of structs.

Structs may contain padding bytes which when copied can leak
uninitialized data across a trusted boundary. In Chrome, this
means previously-written or uninitialized bytes in the GPU process
could leak into the Renderer process.

In the future, we may be able to bring this back by introducing
a concept of safe-to-memcpy structs, or by forking the code so
that Renderer->GPU process uses the unsafe memcpy, and
GPU->Renderer uses to the safe member-by-member version.

An alternative might be to ensure that everything returned from
the WebGPU API has initialized padding, but this invariant is not
trivial to guarantee.

Fixed: chromium:1359098
Change-Id: I91151251d1ab999e0f5552e1efccc472e451cc10
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/102461
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Austin Eng <enga@chromium.org>

generator/templates/dawn/wire/WireCmd.cpp

@@ -64,13 +64,10 @@
         {%- set Optional = "Optional" if member.optional else "" -%}
         WIRE_TRY(provider.Get{{Optional}}Id({{in}}, &{{out}}));
     {%- elif member.type.category == "structure" -%}
-        {%- if member.type.is_wire_transparent -%}
-            static_assert(sizeof({{out}}) == sizeof({{in}}), "Serialize memcpy size must match.");
-            memcpy(&{{out}}, &{{in}}, {{member_transfer_sizeof(member)}});
-        {%- else -%}
-            {%- set Provider = ", provider" if member.type.may_have_dawn_object else "" -%}
-            WIRE_TRY({{as_cType(member.type.name)}}Serialize({{in}}, &{{out}}, buffer{{Provider}}));
-        {%- endif -%}
+        //* Do not memcpy or we may serialize padding bytes which can leak information across a
+        //* trusted boundary.
+        {%- set Provider = ", provider" if member.type.may_have_dawn_object else "" -%}
+        WIRE_TRY({{as_cType(member.type.name)}}Serialize({{in}}, &{{out}}, buffer{{Provider}}));
     {%- else -%}
         {{out}} = {{in}};
     {%- endif -%}