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Each test was creating:
* a non-incremental wrapper script,
* an incremental wrapper script,
* another non-incremental wrapper script.

This third target is actually just needed to depend on the other two, so
refactored to make this more clear. It also makes it easier to address
changing incremental install a GN arg (linked bug).

Bug: 1013688
Change-Id: I530e95a3277aa9a5164e7ad9a02f0f1ec830e556
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1856841
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707243}