bindings: Add a comment to state toV8() can return empty handle
bindings: Add a comment to state toV8() can return empty handle

toV8() can return an empty handle for various reason. For example,
- nullptr is passed.
- Failed to create a wrapper object.
- A given ScriptValue is empty.

Making toV8() return MaybeLocal may not be feasible because they
are used by a lot of places and it's difficult to guarantee that
toV8() fails iff an exception occurred.

BUG=462402

Review URL: https://codereview.chromium.org/1095233002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194095 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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