dawn.node: copy .js files to the right target dir and improve doc for Windows

Because MSVC is a multiconfig toolchain, CMake will output binaries
under build_dir/<config>. Fix the copying of index.js and cts.js so that
they end up in the same directory as dawn.node.

Also update the README with simpler instructions for Windows, using
MSBuild rather than Ninja.

Bug: dawn:1917
Change-Id: Ib147579e794f5080e5de931c40cda1ef0cbb269f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/152960
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/src/dawn/node/CMakeLists.txt b/src/dawn/node/CMakeLists.txt
index d7e39ef..cb6502c 100644
--- a/src/dawn/node/CMakeLists.txt
+++ b/src/dawn/node/CMakeLists.txt
@@ -141,7 +141,7 @@
         TARGET dawn_node POST_BUILD
         COMMAND ${CMAKE_COMMAND} -E copy_if_different
-            "${Dawn_BINARY_DIR}/${file}")
+            "$<TARGET_FILE_DIR:dawn_node>/${file}")
diff --git a/src/dawn/node/README.md b/src/dawn/node/README.md
index 924658d..a3697f6 100644
--- a/src/dawn/node/README.md
+++ b/src/dawn/node/README.md
@@ -37,7 +37,7 @@
 If you don't have those supporting libraries, then you must use the
-`-DDAWN_USE_X11=OFF` flag on Cmake.
+`-DDAWN_USE_X11=OFF` flag on CMake (see below).
 ### Build
@@ -50,13 +50,22 @@
 ninja dawn.node
+On Windows, the steps are similar:
+mkdir <build-output-path>
+cd <build-output-path>
+cmake <dawn-root-path> -DDAWN_BUILD_NODE_BINDINGS=1
+cmake --build . --target dawn_node
 ### Running WebGPU CTS
 1. [Build](#build) the `dawn.node` NodeJS module.
 2. Checkout the [WebGPU CTS repo](https://github.com/gpuweb/cts) or use the one in `third_party/webgpu-cts`.
 3. Run `npm install` from inside the CTS directory to install its dependencies.
-Now you can run CTS:
+Now you can run CTS using our `./tools/run` shell script. On Windows, it's recommended to use MSYS2 (e.g. Git Bash):
 ./tools/run run-cts --bin=<path-build-dir> [WebGPU CTS query]