Previously only JSFunctions seemed to be valid for toJSON, which doesn't
match the ES6 specification that allows any object with [[Call]]
internal method (i.e. any Callable in V8 terminology), including bound
functions and proxies.
BUG=chromium:595738, chromium:535408
R=yangguo@chromium.org
LOG=n
Review URL: https://codereview.chromium.org/1824533002
Cr-Commit-Position: refs/heads/master@{#34913}
