[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.
 
 
+*Notes*:
+
+* `is_debug`:  ClusterFuzz uses release builds by default (`is_debug=false`).
+For ASan builds, both Debug and Release configurations are supported.
+Check a job type of the report for presence of `_debug` suffix.
+
+* `ffmpeg_branding`: For Linux `ffmpeg_branding` should be set to `ChromeOS`.
+For other platforms, use `ffmpeg_branding=Chrome`.
+
 ### 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=is_debug=false use_afl=true is_asan=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="ChromeOS"'
 $ 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=is_debug=false use_libfuzzer=true is_asan=true enable_nacl=false proprietary_codecs=true ffmpeg_branding="ChromeOS"'
 $ 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="ChromeOS"'
 $ 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="ChromeOS"'
 $ 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.