update chromium clang docs

docs/clang.md

 # Clang
 
 [Clang](http://clang.llvm.org/) is a compiler with many desirable features
 (outlined on their website).
 
 Chrome can be built with Clang. It is now the default compiler on Mac and Linux
 for building Chrome, and it is currently useful for its warning and error
 messages on Android and Windows.
 
 See
 [the open bugs](http://code.google.com/p/chromium/issues/list?q=label:clang).
 
 [TOC]
 
 ## Build instructions
 
 Get clang (happens automatically during `gclient runhooks` on Mac and Linux):
 
     tools/clang/scripts/update.py
 
 Only needs to be run once per checkout, and clang will be automatically updated
 by `gclient runhooks`.
 
 Regenerate the ninja build files with Clang enabled. Again, on Linux and Mac,
 Clang is the default compiler.
 
 Run `gn args` and add `is_clang = true` to your args.gn file.
 
-Build: `ninja -C out/Debug chrome`
+Build: `ninja -C out/gn chrome`
 
 ## Reverting to gcc on linux
 
 We don't have bots that test this, but building with gcc4.8+ should still work
 on Linux. If your system gcc is new enough, run `gn args` and add `is_clang =
 false`.
 
 ## Mailing List
 
 http://groups.google.com/a/chromium.org/group/clang/topics
@@ skipping 10 matching lines @@
     to build the plugin.
 1.  Run `ninja -C third_party/llvm-build/Release+Asserts/` to build incrementally.
 1.  Build with clang like described above, but, if you use goma, disable it.
 
 To test the FindBadConstructs plugin, run:
 
     (cd tools/clang/plugins/tests && \
      ./test.py ../../../../third_party/llvm-build/Release+Asserts/bin/clang \
                ../../../../third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so)
 
-These instructions are for GYP which no longer works. Something similar needs
-to be set up for the GN build if you want to do this. For reference, here are
-the old instructions: To run [other plugins](writing_clang_plugins.md), add
-these to your `GYP_DEFINES`:
-
-*   `clang_load`: Absolute path to a dynamic library containing your plugin
-*   `clang_add_plugin`: tells clang to run a specific PluginASTAction
-
-So for example, you could use the plugin in this directory with:
-
-*   `GYP_DEFINES='clang=1 clang_load=/path/to/libFindBadConstructs.so
-    clang_add_plugin=find-bad-constructs' gclient runhooks`
-
 ## Using the clang static analyzer
 
 See [clang_static_analyzer.md](clang_static_analyzer.md).
 
 ## Windows
 
 clang can be used as compiler on Windows. Clang uses Visual Studio's linker and
 SDK, so you still need to have Visual Studio installed.
 
 Things should compile, and all tests should pass. You can check these bots for
 how things are currently looking:
 http://build.chromium.org/p/chromium.fyi/console?category=win%20clang
 
-``` shell
+```shell
 python tools\clang\scripts\update.py
 # run `gn args` and add `is_clang = true` to your args.gn, then...
-ninja -C out\Debug chrome
+ninja -C out\gn chrome
 ```
 
 The `update.py` script only needs to be run once per checkout. Clang will be
 kept up to date by `gclient runhooks`.
 
 Current brokenness:
 
 *   To get colored diagnostics, you need to be running
     [ansicon](https://github.com/adoxa/ansicon/releases).
 *   Debug info does now work, but support for it is new.  If you see something
     not working right, please file a bug and mark it as blocking the
     [clang/win debug info tracking bug](https://crbug.com/636111).
 
 ## Using a custom clang binary
 
-These instructions are for GYP which no longer works. Something similar needs
-to be set up for the GN build if you want to do this. For reference, here are
-the old instructions:
-
-If you want to try building Chromium with your own clang binary that you've
-already built, set `make_clang_dir` to the directory containing `bin/clang`
-(i.e. the directory you ran cmake in, or your `Release+Asserts` folder if you
-use the configure/make build). You also need to disable chromium's clang plugin
-by setting `clang_use_chrome_plugins=0`, as it likely won't load in your custom
-clang binary.
+Set `clang_base_path` in your args.gn to the llvm build directory containing
+`bin/clang` (i.e. the directory you ran cmake). This [must][1] be an absolute
+path. You also need to disable chromium's clang plugin.
 
 Here's an example that also disables debug info and enables the component build
 (both not strictly necessary, but they will speed up your build):
 
-```shell
-GYP_DEFINES="clang=1 fastbuild=1 component=shared_library \
-clang_use_chrome_plugins=0 make_clang_dir=$HOME/src/llvm-build" \
-build/gyp_chromium
+```
+clang_base_path = getenv("HOME") + "/src/llvm-build"
+clang_use_chrome_plugins = false
+is_debug = false
+symbol_level = 1
+is_component_build = true
+is_clang = true  # Implicitly set on Mac, Linux, iOS; needed on Win and Android.
 ```
 
-You can then run `head out/Release/build.ninja` and check that the first to
+You can then run `head out/gn/toolchain.ninja` and check that the first to
 lines set `cc` and `cxx` to your clang binary. If things look good, run `ninja
--C out/Release` to build.
+-C out/gn` to build.
 
 If your clang revision is very different from the one currently used in chromium
 
 *   Check `tools/clang/scripts/update.py` to find chromium's clang revision
-*   You might have to tweak warning flags. Or you could set `werror=` in the
-    line above to disable warnings as errors (but this only works on Linux).
+*   You might have to tweak warning flags.
 
 ## Using LLD
 
 **Experimental!**
 
 LLD is a relatively new linker from LLVM. The current focus is on Windows and
 Linux support, where it can link Chrome approximately twice as fast as gold and
 MSVC's link.exe as of this writing. LLD does not yet support generating PDB
 files, which makes it hard to debug Chrome while using LLD.
 
-Set `use_lld = true` in args.gn. Currently this configuration is only supported
 on Windows.
+Set `use_lld = true` in args.gn.