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This allows overriding it in gn args, which is necessary to use the
correct WebView target on official bots.

Bug: 1054351
Change-Id: I7564595657490faba7b18c9bcc55ede21d3bffc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2187858Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#766543}