This CL refactors the internal representation of JavaScript-exposed
WebAssembly objects to be more like other such objects in V8. By introducing
a new instance type for each of the JS-exposed types, we get more robust
typechecking without using embedder fields (which were previously used
when these objects where instance type JS_API_OBJECT).

In addition to the new instance types, the subclasses X of JSObject
(WasmInstanceObject, WasmMemoryObject, WasmModuleObject, WasmTableObject)
now have appropriate Is##X() methods on Object and are now robust.

BUG=v8:6547
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

Review-Url: https://codereview.chromium.org/2964943002
Cr-Commit-Position: refs/heads/master@{#46475}