Removed v8::Isolate::GetCurrent() and fixed toV8 with empty context.

BUG=369451

Review URL: https://codereview.chromium.org/302973003

git-svn-id: svn://svn.chromium.org/blink/trunk@175279 bbb929c8-8fbe-4397-9dbb-9b2b20218538