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. Usually, having any
JavaScript function on the stack is enough to ensure that; but in the
special case of async compilation, the "success" task is executed
directly from the event loop, so we have to manually enter a context.

Additionally, we need to ensure that such a start function's own context
is properly entered: in addition to setting it as the current context
on the isolate (as the function call sequence is doing), we have to
register it in the list of entered contexts, as if v8::Context::Enter
had been used.

Bug: chromium:1096558
Change-Id: I12679ab49ee764572e3742da24889dcd55c29160
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2292248Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70175}