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The downstream iOS bots import a gn file from $root_build_dir/args.gn
that would not be correctly parsed by //build/gn_helpers.py as:

1.  define some values of type scope (`{ a = "value"}`),
2.  it uses comments in many places (before all variables that are
    defined, above individual items in lists, ...).

To ensure that the file $root_build_dir/args.gn could be parsed, fix
//build/gn_helpers.py to support the scope values and to consider
comments as valid in any location where whitespace would have been.

Bug: 1144711
Change-Id: I0c48167f1bc818dc9f6253287c2c967d13756523
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513217
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823230}