debug_info_test: check that chrome was built with ICF enabled

For both amd64 and arm, when ICF (Identical Code Folding) is enabled,
there is >60,000 drop in the number of unique addresses of text symbols.
We check here that the drop is at least 10,000.

See https://crbug.com/813272#c18 for details.

BUG=chromium:813272
TEST='./debug_info_test.py /build/chell/usr/lib/debug/opt/google/chrome/chrome.debug'
TEST='./debug_info_test.py /build/elm/usr/lib/debug/opt/google/chrome/chrome.debug'

Change-Id: I23d1b48e6547bf3042096bed1c566ec50c5b2d66
Reviewed-on: https://chromium-review.googlesource.com/1053388


Commit-Ready: Rahul Chaudhry <rahulchaudhry@chromium.org>
Tested-by: Rahul Chaudhry <rahulchaudhry@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>