libfuzzer: add documentation recommending how to silence error logging

testing/libfuzzer/getting_started.md

 # Getting Started with libFuzzer in Chrome
 
 *** note
 **Prerequisites:** libFuzzer in Chrome is supported with GN on Linux only. 
 ***
 
 This document will walk you through:
 
 * setting up your build enviroment.
 * creating your first fuzzer.
@@ skipping 86 matching lines @@
 of the biggest testcase in corpus if corpus is not empty. ClusterFuzz takes
 random value in range from `1` to `10000` for each fuzzing session and passes
 that value to libFuzzers. If corpus contains testcases of size greater than
 `max_len`, libFuzzer will use only first `max_len` bytes of such testcases.
 
 
 You can specify custom `max_len` value to be used by ClusterFuzz. For more
 information check out [Maximum Testcase Length] section of the [Efficient Fuzzer
 Guide].
 
+## Remove error message logging

[inferno, 2016/08/30 15:57:39]

s/Remove/Disable noisy

[robert.bradford, 2016/08/30 16:16:51]

Done.

+
+If the code that you are a fuzzing generates error messages when encountering
+incorrect or invalid data then you need to silence those errors in the fuzzer.
+The best way to do that is to override the environment used for logging in your

[mmoroz, 2016/08/30 15:54:00]

I think it's worth to clarify "If the target uses Chromium logging interface,
...". Because some targets (usually third party libraries) have other logging
functions and this trick won't work there. 

[robert.bradford, 2016/08/30 16:16:51]

Done.

+fuzzer:
+
+```cpp
+struct Environment {
+  Environment() {
+    logging::SetMinLogLevel(logging::LOG_FATAL);
+  }
+};
+
+Environment* env = new Environment();
+```
+
 ## Submitting Fuzzer to ClusterFuzz
 
 ClusterFuzz builds and executes all `fuzzer_test` targets in the source tree.
 The only thing you should do is to submit a fuzzer into Chrome.
 
 ## Next Steps
 
 * After your fuzzer is submitted, you should check its [ClusterFuzz status] in
 a day or two.
 * Check the [Efficient Fuzzer Guide] to better understand your fuzzer
@@ skipping 15 matching lines @@
 
 
 [Address Sanitizer]: http://clang.llvm.org/docs/AddressSanitizer.html
 [Memory Sanitizer]: http://clang.llvm.org/docs/MemorySanitizer.html
 [Undefined Behavior Sanitizer]: http://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html
 [ClusterFuzz status]: clusterfuzz.md#Status-Links
 [crbug/598448]: https://bugs.chromium.org/p/chromium/issues/detail?id=598448
 [Efficient Fuzzer Guide]: efficient_fuzzer.md
 [Maximum Testcase Length]: efficient_fuzzer.md#Maximum-Testcase-Length
 [url_parse_fuzzer.cc]: https://code.google.com/p/chromium/codesearch#chromium/src/testing/libfuzzer/fuzzers/url_parse_fuzzer.cc