[emscripten] Fix Cmake parse arguments to build samples.

- I didn't realize I was using the parse arguments incorrectly and
  this actually caused the samples not to build. (I added this towards
  the end). Unfortunately, because I didn't clean my build output
  directory, I didn't realize I was just hitting a cached build.

Change-Id: I351fba0b78454f068914f1bc43410042863d8ddd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/201757
Auto-Submit: Loko Kung <lokokung@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index c35a7bb..77c0743 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -290,6 +290,7 @@
 message(STATUS "Dawn build Vulkan backend: ${DAWN_ENABLE_VULKAN}")
 message(STATUS "Dawn build OpenGL backend: ${DAWN_ENABLE_DESKTOP_GL}")
 message(STATUS "Dawn build OpenGL ES backend: ${DAWN_ENABLE_OPENGLES}")
+message(STATUS "Dawn build Emscripten: ${DAWN_ENABLE_EMSCRIPTEN}")
 message(STATUS "Dawn build Null backend: ${DAWN_ENABLE_NULL}")
 
 message(STATUS "Dawn enable SPIR-V validation: ${DAWN_ENABLE_SPIRV_VALIDATION}")
diff --git a/src/dawn/samples/CMakeLists.txt b/src/dawn/samples/CMakeLists.txt
index cb1ef0f..8eee09c 100644
--- a/src/dawn/samples/CMakeLists.txt
+++ b/src/dawn/samples/CMakeLists.txt
@@ -64,9 +64,9 @@
 
 function(Sample)
     cmake_parse_arguments(PARSE_ARGV 0 arg
-        ""
-        "NAME;ENABLE_EMSCRIPTEN;SOURCES"
-        ""
+        "ENABLE_EMSCRIPTEN"
+        "NAME"
+        "SOURCES"
     )
 
     # Skip targets that shouldn't be built with Emscripten.