Add an ASSERT about cross-world wrapper leakage into ScriptValue

- This CL adds an ASSERT to ScriptValue to verify that ScriptValue::v8Value() is used in the same world that created the ScriptValue.

- To insert the check, we need to get a current world in ScriptValue::v8Value(). For that goal, this CL makes sure that we're in some context when calling ScriptValue::v8Value().

BUG=341032

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175837

Review URL: https://codereview.chromium.org/314953005

git-svn-id: svn://svn.chromium.org/blink/trunk@175944 bbb929c8-8fbe-4397-9dbb-9b2b20218538