MIPS: Improve write barriers in optimized code.

Port r21630 (a21ff10)

Original commit message:
Use a cheaper RecordWriteForMap() to update the
write barrier for maps. And skip the value check
in RecordWriteField() when we statically know that
the value is in new space (and therefore has "pointers
to here are interesting" flag set).

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/315653002

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21657 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Add API support for passing a C++ function as a microtask callback

This allows embedders to enqueue microtasks without having any v8::Context
handy, as happens in Blink in some cases (such as DOM mutations due to editing
triggering MutationObservers).

LOG=Y
R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/306053003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21658 ce2b1a6d-e550-0410-aec6-3dcde31c8c00