debugd: add special iwlwifi_dump log key

This log key will contain information which is both very large
and pretty much useless for users to look at if they want to
understand what is being sent in the system information dialog.
Instead, we want to check for logs with this key and replace them
with an explanatory string.

BUG=b:65121142
TEST=emerge

Change-Id: Ia8da7843b671e239139e9786d89f8e1dbf48980b
Reviewed-on: https://chromium-review.googlesource.com/810016


Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>