Allow microtasks to throw exceptions and handle them gracefully

If the embedder calls V8::TerminateExecution while we're running microtasks, bail out
and clear any pending microtasks.

All other exceptions are simply swallowed. No current Blink or V8 microtasks throw, this
just ensures something sane happens if another embedder decides to pass a throwing
microtask (or if ours unexpectedly throw due to, e.g., stack exhaustion).

BUG=371566
LOG=Y
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/294943009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21574 ce2b1a6d-e550-0410-aec6-3dcde31c8c00