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So that we don't accidentally add new targets that don't set
the variable (which is required for compile deps and wrapper scripts
to work properly).

Change-Id: I8541f2fe83bd7ced7a9d215b415c009abda37468
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2003568Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Eric Stevenson <estevenson@chromium.org>
Auto-Submit: Eric Stevenson <estevenson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734040}