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
Simplify, speed-up correct-context ObjectObserve calls

The original patch which ensured that Object.observe did allocations in the correct context regressed performance about 12%. This patch gets back most of that (about 11%) by simply returning the correct function which is then directly callable from JS, rather than by making the call from the runtime function. A side-effect is that their implementation is shorter.

LOG=Y
BUG=NONE
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/307543008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21575 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'