binding: Retires ExceptionState::throwIfNeeded().

It's more convenient for call sites to let ExceptionState
throw by default than to explicitly call to throwIfNeeded().

This prevents a mistake that a call site forgets to throw.
ExceptionState now looks a kind of exception scope.

BUG=

Review-Url: https://codereview.chromium.org/2272613003
Cr-Commit-Position: refs/heads/master@{#415139}
51 files changed