Attempting to upload a uniform value to WebGL from a multithreaded WebAssembly program results in the following assertion failure in a debug build, or a build with dcheck_always_on=true:

[87532:775:1113/130013.706764:FATAL:array_buffer_view.h(59)] Check failed: !IsShared(). 
0   libbase.dylib                       0x000000010b9e1b1f base::debug::StackTrace::StackTrace(unsigned long) + 31
1   libbase.dylib                       0x000000010b8d8f6f logging::LogMessage::~LogMessage() + 223
2   libblink_modules.dylib              0x000000011bb2e274 blink::FlexibleArrayBufferView::BaseAddressMaybeOnStack() const + 116
3   libblink_modules.dylib              0x000000011bb22050 blink::WebGL2RenderingContextBase::uniform4fv(blink::WebGLUniformLocation const*, blink::TypedFlexibleArrayBufferView<WTF::Float32Array> const&, unsigned int, unsigned int) + 48
4   libblink_modules.dylib              0x000000011b491da5 blink::webgl2_rendering_context_v8_internal::uniform4fv1Method(v8::FunctionCallbackInfo<v8::Value> const&) + 741
5   libv8.dylib                         0x0000000109761319 v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo*) + 937
6   libv8.dylib                         0x000000010975fa22 v8::internal::MaybeHandle<v8::internal::Object> v8::internal::(anonymous namespace)::HandleApiCallHelper<false>(v8::internal::Isolate*, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::FunctionTemplateInfo>, v8::internal::Handle<v8::internal::Object>, v8::internal::BuiltinArguments) + 978
7   libv8.dylib                         0x000000010975e10a v8::internal::Builtin_Impl_HandleApiCall(v8::internal::BuiltinArguments, v8::internal::Isolate*) + 554
8   libv8.dylib                         0x000000010975dcaa v8::internal::Builtin_HandleApiCall(int, unsigned long*, v8::internal::Isolate*) + 122
9   libv8.dylib                         0x000000010a5ce892 v8_Default_embedded_blob_ + 2903506


FlexibleArrayBuffer must allow shared ArrayBuffers. The WebIDL should also be changed to use AllowShared where FlexibleArrayBuffer is used, though it seems the AllowShared checks aren't catching this.