[wasm] Make an "incumbent context" available for module instantiation

A Wasm module's start function might be imported from JavaScript, and
as such might contain calls to Blink. For such a case, we must make
sure that an "incumbent context" is available.
See microtask queue handling for a similar example.

Bug: chromium:1096558
Change-Id: I2e3c0fc20d4e3581e490822c3ac63ce2c5e1e990
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284982Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68760}

src/wasm/module-instantiate.cc

@@ -4,6 +4,7 @@
 
 #include "src/wasm/module-instantiate.h"
 
+#include "src/api/api.h"
 #include "src/asmjs/asm-js.h"
 #include "src/logging/counters.h"
 #include "src/numbers/conversions-inl.h"
@@ -641,10 +642,15 @@ bool InstanceBuilder::ExecuteStartFunction() {
   if (start_function_.is_null()) return true;  // No start function.
 
   HandleScope scope(isolate_);
+  // We have to make sure that an "incumbent context" is available in case
+  // the start function calls out to Blink.
+  HandleScopeImplementer* hsi = isolate_->handle_scope_implementer();
+  hsi->EnterContext(isolate_->context());
   // Call the JS function.
   Handle<Object> undefined = isolate_->factory()->undefined_value();
   MaybeHandle<Object> retval =
       Execution::Call(isolate_, start_function_, undefined, 0, nullptr);
+  hsi->LeaveContext();
 
   if (retval.is_null()) {
     DCHECK(isolate_->has_pending_exception());