Fix wasm_context wrong usage.

Previous usage was getting wrong address on Mips.

TEST=test-wasm-trap-position/RunWasmTurbofan_IllegalLoad

Bug: 
Change-Id: I325ada99540f01ae509dfbfe3c0e55693dacca19
Reviewed-on: https://chromium-review.googlesource.com/859457Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50474}

test/cctest/wasm/wasm-run-utils.cc

@@ -118,7 +118,7 @@ Handle<JSFunction> TestingModuleBuilder::WrapCode(uint32_t index) {
                              : WasmCodeWrapper(function_code_[index]);
   byte* context_address =
       test_module_.has_memory
-          ? reinterpret_cast<byte*>(instance_object_->wasm_context())
+          ? reinterpret_cast<byte*>(instance_object_->wasm_context()->get())
           : nullptr;
   Handle<Code> ret_code = compiler::CompileJSToWasmWrapper(
       isolate_, &test_module_, code, index, context_address,