Replace dawn::CallbackUserdata with void*

This initial reasoning for having a u64 userdata was to be able to pack
two u32s in a single userdata but that was never used, and made a bunch
of code uglier than it should.

BUG=dawn:160

Change-Id: Ia0d20bc23f09f5d8f3748ca4edd1a331604f2ba8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7561
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/tests/unittests/validation/QueueSubmitValidationTests.cpp

@@ -24,9 +24,8 @@ class QueueSubmitValidationTest : public ValidationTest {
 static void StoreTrueMapWriteCallback(DawnBufferMapAsyncStatus status,
                                       void*,
                                       uint64_t,
-                                      DawnCallbackUserdata userdata) {
-    bool* userdataPtr = reinterpret_cast<bool*>(static_cast<intptr_t>(userdata));
-    *userdataPtr = true;
+                                      void* userdata) {
+    *static_cast<bool*>(userdata) = true;
 }
 
 // Test submitting with a mapped buffer is disallowed
@@ -56,8 +55,7 @@ TEST_F(QueueSubmitValidationTest, SubmitWithMappedBuffer) {
 
     // Map the buffer, submitting when the buffer is mapped should fail
     bool mapWriteFinished = false;
-    DawnCallbackUserdata userdata = static_cast<DawnCallbackUserdata>(reinterpret_cast<intptr_t>(&mapWriteFinished));
-    buffer.MapWriteAsync(StoreTrueMapWriteCallback, userdata);
+    buffer.MapWriteAsync(StoreTrueMapWriteCallback, &mapWriteFinished);
     queue.Submit(0, nullptr);
     ASSERT_TRUE(mapWriteFinished);