Windows performance.

[brendandahl]

As we found out in #774 busy waiting improves windows performance but also causes crashes. Some notes found so far:

• Seems to crash in libjs at http://mxr.mozilla.org/mozilla-central/source/js/src/ctypes/CTypes.cpp#6130 because the JSContext is 0xdddddddd (freed). This most commonly comes from our code at https://github.com/mozilla/r2d2b2g/blob/release-5.0/android-tools/adb-bin/usb_windows.cpp#L437 though I've seen it crash in other spots as well.
• bholley pointed me to a patch(https://bugzilla.mozilla.org/show_bug.cgi?id=915613) that will change how jscontext is found. Unfortunately, with this patch applied our code dies on the assertion JS_ASSERT(rt->contextList.getFirst() == rt->contextList.getLast());

Waiting to hear back for more info if this is our problem our something with the patch.

[brendandahl]

Reproduced the bug with a tiny library, tracking in https://bugzilla.mozilla.org/show_bug.cgi?id=925604