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This was removed from the declare_args section some time ago, but for
AOSP system implementors it may be desirable to manually control it.
Re-add it as an argument, and document how to use it in the WebView AOSP
build guide.

Fixed: 1034192
Change-Id: Ic5f890f2ad3677bb76ce87951ee0d7d3f20551b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976331Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730188}