GN configs for ipc fuzzer.

docs/ipc_fuzzer.md

 # IPC Fuzzer
 
 A chromium IPC fuzzer is under development by aedla and tsepez. The fuzzer lives
 under `src/tools/ipc_fuzzer/` and is running on ClusterFuzz. A previous version
 of the fuzzer was a simple bitflipper, which caught around 10 bugs. A new
 version is doing smarter mutations and generational fuzzing. To do so, each
 `ParamTraits<Type>` needs a corresponding `FuzzTraits<Type>`. Feel free to
 contribute.
 
 [TOC]
 
 ## Working with the fuzzer
 
 ### Build instructions
 
-*   add `enable_ipc_fuzzer=1` to `GYP_DEFINES`
+*   Run `gn args` and add `enable_ipc_fuzzer = true` to your args.gn. If you use
+    GYP, add `enable_ipc_fuzzer=1` to `GYP_DEFINES`.
 *   build `ipc_fuzzer_all` target
 *   component builds are currently broken, sorry
 *   Debug builds are broken; only Release mode works.
 
 ### Replaying ipcdumps
 
 *   `tools/ipc_fuzzer/scripts/play_testcase.py path/to/testcase.ipcdump`
 *   more help: `tools/ipc_fuzzer/scripts/play_testcase.py -h`
 
 ### Listing messages in ipcdump
@@ skipping 29 matching lines @@
 with the `ipc_fuzzer/play_testcase.py` convenience script.
 
 ### ipcdump mutator / generator
 
 Lives under `ipc_fuzzer/fuzzer`. This is the code that runs on ClusterFuzz. It
 uses `FuzzTraits<Type>` to mutate ipcdumps or generate them out of thin air.
 
 ## Problems, questions, suggestions
 
 Send them to mbarbella@chromium.org.