Make iterator debugging off by default

The cost of iterator debugging is too great so it should be off by
default. It increases build times and sometimes increases run times by
multiple orders of magnitude. I am not aware of it finding bugs
recently.

Bug: 904034
Change-Id: If3d1ea04a6213ed189cbfeb35bc0980283123acb
Reviewed-on: https://chromium-review.googlesource.com/c/1330892
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607480}

build/config/BUILD.gn

@@ -32,8 +32,11 @@ declare_args() {
   # 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
+  # significantly slow execution - two orders of magnitude slowdown has been
+  # seen (crbug.com/903553) and iterator debugging also slows builds by making
+  # generation of snapshot_blob.bin take ~40-60 s longer. Therefore this
+  # defaults to off.
+  enable_iterator_debugging = false
 }
 
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