[libfuzzer] Add "Symbolization" and "Debugging" sections to reproducing.md page.

testing/libfuzzer/reproducing.md

 # Reproducing ClusterFuzz bugs locally
 
 ClusterFuzz will report bugs in the bug tracker in the following form:
 
 ```
 Detailed report: https://cluster-fuzz.appspot.com/testcase?key=...
 
 Fuzzer: libfuzzer_media_pipeline_integration_fuzzer
 Job Type: libfuzzer_chrome_asan
 Platform Id: linux
@@ skipping 68 matching lines @@
 
 ### Reproducing LibFuzzer + UBSan bugs
 
 ```bash
 $ gn gen out/libfuzzer '--args=is_debug=false use_libfuzzer=true is_ubsan_security=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="ChromeOS"'
 $ ninja -C out/libfuzzer $FUZZER_NAME
 $ export UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1
 $ out/libfuzzer/$FUZZER_NAME /path/to/repro
 ```
 
+### Symbolization
+
+Memory tools (ASan, MSan, UBSan) use [llvm-symbolizer] binary from the Clang
+distribution to symbolize the stack traces. To get a symbolized crash report,
+make sure `llvm-symbolizer` is in `PATH` or provide it in separate

[Nico, 2016/08/29 12:04:13]

Mention that llvm-symbolizer is in third_party/llvm-build/Release+Asserts/bin
and/or that it's copied to the build dir at build time (and make the example
point there)

[mmoroz, 2016/08/29 12:40:00]

Done.

+`ASAN_SYMBOLIZER_PATH` environment variable:
+
+```bash
+$ export ASAN_SYMBOLIZER_PATH=/path/to/llvm_build/bin/llvm-symbolizer
+$ out/libfuzzer/$FUZZER_NAME /path/to/repro
+```
+
+The same approach works for `MSAN_SYMBOLIZER_PATH` and `UBSAN_SYMBOLIZER_PATH`.
+
+Additional information regarding symbolization is available in sanitizers
+documentation: [AddressSanitizerCallStack].
+
+
+### Debugging
+
+Please checkout [AddressSanitizerAndDebugger] page for some tips on debugging of

[Nico, 2016/08/29 12:04:13]

s/checkout/look at/ since "checkout" sounds like `svn checkout`. (Also,
"checkout" is the noun, while the verb is "check out" with a space).

Doesn't abort_on_error work in ASAN_OPTIONS as well?

[mmoroz, 2016/08/29 12:40:00]

Thanks for the explanation! My bad.

abort_on_error works for ASan too. My intention was to do not write about that,
since the link contains that information. But probably it's worth to be
mentioned here. Done.

+binaries built with ASan. For binaries built with MSan,
+`MSAN_OPTIONS=abort_on_error=1` makes gdb to stop after the report.
+
+
+[AddressSanitizerAndDebugger]: https://github.com/google/sanitizers/wiki/AddressSanitizerAndDebugger
+[AddressSanitizerCallStack]: https://github.com/google/sanitizers/wiki/AddressSanitizerCallStack
+[llvm-symbolizer]: http://llvm.org/docs/CommandGuide/llvm-symbolizer.html