Add gn Blink GC plugin options

third_party/WebKit/Source/config.gni

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/ui.gni")
 import("//third_party/WebKit/public/features.gni")
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 
 declare_args() {
   # Set to true to enable the clang plugin that checks the usage of the Blink
   # garbage-collection infrastructure during compilation.
   blink_gc_plugin = true
 
+  # Set to true to have the clang Blink GC plugin emit class graph (in JSON)
+  # with typed pointer edges; for debugging or other (internal) uses.
+  blink_gc_plugin_option_do_dump_graph = false

[Nico, 2016/06/21 20:53:13]

you can have declare_args in BUILD.gn files, so if this is only needed in one
file (it looks like it is), then this should be in that file (random example:
https://cs.chromium.org/chromium/src/skia/BUILD.gn?rcl=0&l=23)

[sof, 2016/06/22 10:58:04]

I've moved the GC plugin ones to Source/BUILD.gn , if we don't want to keep
Source/ configurables in one place.

+
+  # Set to true to have the clang Blink GC plugin additionally check if
+  # a class has an empty destructor which would be unnecessarily invoked
+  # when finalized.
+  blink_gc_plugin_option_warn_unneeded_finalizer = false
+
+  # Clang plugin installation directory; the default is
+  # to use the build tree "third_party/llvm-build/Release+Asserts/lib"
+  # directory.
+  blink_gc_plugin_lib_path = "//third_party/llvm-build/Release+Asserts/lib/"

[Nico, 2016/06/21 20:53:13]

Hm, the plugin usually only works with clangs build at the same revision --
should we have a "local_clang_build_path" thingy that's used for the clang
compiler, the normal style plugin, and the blink plugin?

[sof, 2016/06/21 21:12:39]

I think it makes sense to broaden the configurable option like that; I haven't
investigated (yet) why https://codereview.chromium.org/1052033006 singled out
libBlinkGCPlugin

[sof, 2016/06/22 10:58:04]

Didn't go the whole hog, but added clang_plugin_lib_path to clang.gni

+
   # If true, doesn't compile debug symbols into webcore reducing the
   # size of the binary and increasing the speed of gdb.
   remove_webcore_debug_symbols = false
 
   # If true, enables blink logging macros (WTF_LOG and friends) unconditionally.
   # When false, blink logging macros are enabled only if assertions are enabled.
   blink_logging_always_on = false
 
   # If true, defaults image interpolation to low quality.
   use_low_quality_image_interpolation = is_android
@@ skipping 56 matching lines @@
 }
 
 # feature_defines_string -------------------------------------------------------
 
 # Convert the list to a space-separated string for passing to scripts.
 # This would be the equivalent of passing '<(feature_defines)' in GYP.
 feature_defines_string =
     exec_script("build/gn_list_to_space_separated_string.py",
                 feature_defines_list,
                 "trim string")