This commit adds a TryGetCurrent() method to the v8::Isolate class.

The motivation for adding this method this is that in Node.js we've run
into situations where we need to check if there is a current
Isolate and we are using GetCurrent() for this. The issue is that for a
debug build of Node.js, the debug check in GetCurrent() will cause a
failure.

The suggestion in this changeset is to allow getting the current
Isolate, or null if one does not exist, without any checks.

Change-Id: I01676e4bcdbe86da0496f5df1982d14eb1c9ebf8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2910630Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75004}