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Adds functionality to create_installer_archive.py to generate a .d file
that includes all of the files that it packages into chrome[.packed].7z.
This is done by tracking the source of all the files that are copied
into the staging dir that is given to 7za to package the .7z file.

The depfile is then used in a gyp 'depfile' to correctly specify the
implicit dependencies for the mini_installer target. Previously it used
a fake output of 'xxx2.out' to cause it to rebuild every time (which is
what we're trying to avoid in this change).

Additionally, switch from a rule to an action in gyp -- gyp currently
only supports 'depfile' on actions, and as there's only one file being
built here, they're equivalent.

BUG=342974,451499

Review URL: https://codereview.chromium.org/887673003

Cr-Commit-Position: refs/heads/master@{#314015}