Update build instructions for Linux and Windows.

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
@@ skipping 15 matching lines @@
 
 If you use gyp: `GYP_DEFINES=clang=1 build/gyp_chromium`
 
 If you use gn, run `gn args` and add `is_clang = true` to your args.gn file.
 
 Build: `ninja -C out/Debug 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, use this to build with gcc if you
-don't want to build with clang:
-
-    GYP_DEFINES=clang=0 build/gyp_chromium
+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
 
 ## Using plugins
 
 The
 [chromium style plugin](http://dev.chromium.org/developers/coding-style/chromium-style-checker-errors)
 is used by default when clang is used.
 
 If you're working on the plugin, you can build it locally like so:
 
 1.  Run `./tools/clang/scripts/update.py --force-local-build --without-android`
     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)
 
 To run [other plugins](writing_clang_plugins.md), add these to your
-`GYP_DEFINES`:
+`GYP_DEFINES` (this is not currently set up in GN):
 
 *   `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
@@ skipping 60 matching lines @@
 **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.
 
 If you use gyp, you can enable it with `GYP_DEFINES=lld=1`. If you use gn, set
 `use_lld = true` in args.gn. Currently this configuration is only supported on
 Windows.