Add gn Blink GC plugin options

build/config/clang/clang.gni

Index: build/config/clang/clang.gni
diff --git a/build/config/clang/clang.gni b/build/config/clang/clang.gni
index 610dd4118a66c8fe777706cf8234d33a07d78900..0f1ce2ab265bc317641240c32620f2f49b4c8956 100644
--- a/build/config/clang/clang.gni
+++ b/build/config/clang/clang.gni
@@ -8,4 +8,8 @@ declare_args() {
   # Indicates if the build should use the Chrome-specific plugins for enforcing
   # coding guidelines, etc. Only used when compiling with Clang.
   clang_use_chrome_plugins = is_clang && !is_nacl && !use_xcode_clang
+
+  # Local clang plugin installation directory, re-configurable to accommodate
+  # distributed compilation setups.

[Nico, 2016/06/22 14:31:47]

Is it useful to switch plugin path independent from compiler path though? I'd
expect that if you set this, the plugin will just crash if it runs with the
pinned compiler.

(If this is useful to you, I'm not opposed to it, but I don't understand how it
can be useful yet.)

[sof, 2016/06/22 14:39:49]

I'm not using it, just carrying over pre-existing functionality.

But if you arrange for a suitable PATH, it should work.

+  clang_plugin_lib_path = "//third_party/llvm-build/Release+Asserts/lib"
 }