wtf/Assertions.h has BACKTRACE(), but it seems to fail to decode some symbols.
Chromium's base/debug/stack_trace.h works better at least on Linux. One caveat is
that unless explicitly add base.gyp:base in core.gyp, link fails on
component=shared_library(GYP) build.
printf() is usable in Blink, but fprintf(stderr, ...) looks better if used with backtrace.