Without --harmony-function-tostring, anything other than a JSFunction
or JSBoundFunction throw when Function.prototype.toString is called on
them. But with the toString revision, anything callable allows toString
(and for non-Functions returns the good old "function () { [native code] }"
string).

Bug: v8:7484
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3540e213a40992151761b59666fe36e0510da908
Reviewed-on: https://chromium-review.googlesource.com/932825
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51489}