CrashReporter: Capture backtrace progress callback's arguments by value

Previously we captured them by reference, but when the deferred code
finally runs from an event loop, the references may be stale.
In that case, value and max of the progressbar are set with random
numbers.
If we were especially unlucky, the `frame_count` turned into a negative
int, and would crash at `VERIFY(min <= max)`.

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