Buffer: Validate the offset is aligned to 8

This is to match the upstream WebGPU spec.

Bug: dawn:445

Change-Id: I1a511ed9a2a04c7b95368ce724d69c128158f097
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/29360
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/Buffer.cpp

diff --git a/src/dawn_native/Buffer.cpp b/src/dawn_native/Buffer.cpp
index 3b2b750..519b0e7 100644
--- a/src/dawn_native/Buffer.cpp
+++ b/src/dawn_native/Buffer.cpp
@@ -403,8 +403,8 @@
         *status = WGPUBufferMapAsyncStatus_Error;
         DAWN_TRY(GetDevice()->ValidateObject(this));
 
-        if (offset % 4 != 0) {
-            return DAWN_VALIDATION_ERROR("offset must be a multiple of 4");
+        if (offset % 8 != 0) {
+            return DAWN_VALIDATION_ERROR("offset must be a multiple of 8");
         }
 
         if (size % 4 != 0) {
@@ -448,6 +448,10 @@
     }
 
     bool BufferBase::CanGetMappedRange(bool writable, size_t offset, size_t size) const {
+        if (offset % 8 != 0 || size % 4 != 0) {
+            return false;
+        }
+
         if (size > mMapSize || offset < mMapOffset) {
             return false;
         }