[libfuzzer] Update GN flags for reproducing bugs.

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 25 matching lines @@
    string specified after the "Fuzzer :" in the report, but *without* the
    "libfuzzer_" or "afl_" prefix. In this case, the `$FUZZER_NAME` is
    "media_pipeline_integration_fuzzer".
 
 3. Follow the steps in one of the subsequent sections (from a chromium
    checkout).  The string specified after the "Job Type: " will be either
    `afl_chrome_asan`, `libfuzzer_chrome_asan`, `libfuzzer_chrome_msan`, or
    `libfuzzer_chrome_ubsan`, indicating which one to use.
 
 
+*Note*: ClusterFuzz uses release builds by default, so it may be worth adding
+`is_debug=false` to your GN args if you are having trouble reproducing a
+particular report. For ASan builds both Debug and Release configurations are
+supported. Check a job type of the report for presence of `_debug` suffix.
+
+
 ### Reproducing AFL + ASan bugs
 ```bash
-$ gn gen out/afl '--args=use_afl=true is_asan=true enable_nacl=false proprietary_codecs=true'
+$ gn gen out/afl '--args=use_afl=true is_asan=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="Chrome"'

[inferno, 2016/08/26 14:45:26]

Most devs will be using is_debug=false, so lets make it default for asan too.
and then fix your above comment.

Also, add point about ffmpeg_branding=ChromeOS for linux, ffmpeg_branding=Chrome
for other platforms.

[mmoroz, 2016/08/26 16:35:59]

Done.

 $ ninja -C out/afl $FUZZER_NAME
 $ out/afl/$FUZZER_NAME < /path/to/repro
 ```
 
 ### Reproducing LibFuzzer + ASan bugs
 
 ```bash
-$ gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true enable_nacl=false proprietary_codecs=true'
+$ gn gen out/libfuzzer '--args=use_libfuzzer=true is_asan=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="Chrome"'
 $ ninja -C out/libfuzzer $FUZZER_NAME
 $ out/libfuzzer/$FUZZER_NAME /path/to/repro
 ```
 
 ### Reproducing LibFuzzer + MSan bugs
 
 ```bash
 # The gclient sync is necessary to pull in instrumented libraries.
 $ GYP_DEFINES='msan=1 use_prebuilt_instrumented_libraries=1' gclient sync
-$ gn gen out/libfuzzer '--args=use_libfuzzer=true is_msan=true msan_track_origins=2 use_prebuilt_instrumented_libraries=true enable_nacl=false proprietary_codecs=true'
+$ gn gen out/libfuzzer '--args=is_debug=false use_libfuzzer=true is_msan=true msan_track_origins=2 use_prebuilt_instrumented_libraries=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="Chrome"'
 $ ninja -C out/libfuzzer $FUZZER_NAME
 $ out/libfuzzer/$FUZZER_NAME /path/to/repro
 ```
 
 ### Reproducing LibFuzzer + UBSan bugs
 
 ```bash
-$ gn gen out/libfuzzer '--args=use_libfuzzer=true is_ubsan_security=true enable_nacl=false proprietary_codecs=true'
+$ gn gen out/libfuzzer '--args=is_debug=false use_libfuzzer=true is_ubsan_security=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="Chrome"'
 $ ninja -C out/libfuzzer $FUZZER_NAME
 $ export UBSAN_OPTIONS=halt_on_error=1:print_stacktrace=1
 $ out/libfuzzer/$FUZZER_NAME /path/to/repro
 ```
 
-*Note*: ClusterFuzz uses release builds by default, so it may be worth adding
-"is_debug=false" to your GN args if you are having trouble reproducing a
-particular report.