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I was going to rename set_default_check_params() to
set_default_check_params_for_testing(), but then realized we don't
need this at all.
Then asargent@ pointed out that we don't even need the member
default_params_ in ExtensionUpdater.

One use of this was to specify check_blacklist (http://crrev.com/11339047),
however, later this was removed (http://crrev.com/23591050), but the
code stayed around.

Cleaning this up to remove confusion.

BUG=None
Test=unit_tests and browser_tests compiles under ChromeOS.

Review URL: https://codereview.chromium.org/1630903003

Cr-Commit-Position: refs/heads/master@{#371524}