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The test runner clears application data before each test it runs, as well as
after it has run all tests. However, some suites drop useful files in the
application data directory. This provides a mechanism for retrieving those
files from the device.

BUG=489713

Review URL: https://codereview.chromium.org/1165623003

Cr-Commit-Position: refs/heads/master@{#333768}