crash-reporter: remove gflags dependency

We are switching to using chromeos/flag_helper.h instead to standardize the
code everywhere.

BUG=chromium:402631
TEST=`FEATURES=test emerge-panther crash-reporter`
TEST=`test_that -b panther <ip> e:logging_.*`

Change-Id: I62b911a54f89d853235d0540460cbea119f66e9e
Reviewed-on: https://chromium-review.googlesource.com/212140


Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Steve Fung <stevefung@chromium.org>
Commit-Queue: Steve Fung <stevefung@chromium.org>