UI hangs are now being caught again on Windows

[Gabriele Svelto]

Hello all,
bug 1716946 [1] just landed and it re-enables capturing hangs of both
the main and child processes if they become unresponsive. Hang detection
was enabled when I first introduced the Windows Error Reporting
interceptor but we turned off the functionality because the hangs were
causing confusion during triage as they got mixed up with the crashes
(see bug 1718226 [2] and bug 1709423 [3]). For the time being this will
still be the case as server-side support for flagging hangs needs to
land only after the new crash reports start flowing in (bug 1826703 [4]).

This is to say don't worry if you see an uptick of crashes under new
signatures in the nightly channel, especially if the crash reason is
`EXCEPTION_BREAKPOINT` but the point of failure is not an exception
(such as a call to `MOZ_CRASH()` or `NS_DebugBreak()`). When in doubt
open the crash annotations tab and check if the "Hang" annotation is
present, if it is then it's a hang, not a crash. You'll need protected
data access to check it, if you don't NI? me and I'll have a look for you.

Gabriele

[1] Investigate why hanging the main process' main thread doesn't
eventually lead to a crash
https://bugzilla.mozilla.org/show_bug.cgi?id=1716946
[2] Do not capture crashes intercepted via Windows Error Reporting that
aren't flagged as fatal
https://bugzilla.mozilla.org/show_bug.cgi?id=1718226
[3] some crash signatures on Windows 7 and 8.1 with unexpected function
in frame #0 of crashing thread
https://bugzilla.mozilla.org/show_bug.cgi?id=1709423
[3] Flag hangs so that we can tell them apart from crashes
https://bugzilla.mozilla.org/show_bug.cgi?id=1826703