Fix ObjectNotifierPerformChange leak after r21126

Due to overlapping names of natives and runtime functions, the wrong
context was used for Notifier.prototype.performChange. The leak test
has been augmented to properly cover the leaky case, and the test
now passes.

Also tightened up type checks in runtime.cc and removed Object.observe
functions from knownIssues in fuzz-natives-part2.js.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/264793015

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21129 ce2b1a6d-e550-0410-aec6-3dcde31c8c00