Rename iterator debugging flag in GN.

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 60f4d1b120aadcd5f94bbf6a2708ca183ba9b06c..d72d4044f3ec3c0a4bcb72bbdde45e0c289dd634 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -12,10 +12,16 @@ import("//build/module_args/v8.gni")
 import("//build/toolchain/goma.gni")
 
 declare_args() {
-  # When set, turns off the (normally-on) iterator debugging and related stuff
-  # that is normally turned on for Debug builds. These are generally useful for
-  # catching bugs but in some cases may cause conflicts or excessive slowness.
-  disable_iterator_debugging = false
+  # When set (the default) enables C++ iterator debugging in debug builds.
+  # Iterator debugging is always off in release builds (technically, this flag
+  # affects the "debug" config, which is always available but applied by
+  # default only in debug builds).
+  #
+  # Iterator debugging is generally useful for catching bugs. But it can
+  # introduce extra locking to check the state of an iterator against the state
+  # of the current object. For iterator- and thread-heavy code, this can
+  # significantly slow execution.
+  enable_iterator_debugging = true
 
   # Set to true to not store any build metadata, e.g. ifdef out all __DATE__
   # and __TIME__. Set to 0 to reenable the use of these macros in the code
@@ -299,13 +305,12 @@ config("debug") {
   }
 
   if (is_win) {
-    if (disable_iterator_debugging) {
-      # Iterator debugging is enabled by the compiler on debug builds, and we
-      # have to tell it to turn it off.
+    if (!enable_iterator_debugging) {
+      # Iterator debugging is enabled by default by the compiler on debug
+      # builds, and we have to tell it to turn it off.
       defines += [ "_HAS_ITERATOR_DEBUGGING=0" ]
     }
-  } else if (is_linux && !is_android && current_cpu == "x64" &&
-             !disable_iterator_debugging) {
+  } else if (is_linux && current_cpu == "x64" && enable_iterator_debugging) {
     # Enable libstdc++ debugging facilities to help catch problems early, see
     # http://crbug.com/65151 .
     # TODO(phajdan.jr): Should we enable this for all of POSIX?