Speed up final phase of compare_build_artifacts.py

On the Windows bot, this step takes 20 minutes, half of it in the second
half of the script where compare_deps() runs. The win whitelist contains
3 files with the same deps (which are virtually "all built files"), so
we spend 10 minutes computing the same information 3 times. Cache it.

On Mac, the step takes 25 minutes, and there we list almost all executables
in the whitelist file, many also depending on virtually all files
(performance_browser_tests, interactive_ui_tests, extensions_browsertests, ...).
This should help the step on that bot as well.

Bug: 314403
Change-Id: I3c0d5a62a3e430a031c6637516c070d9326ec7c5
Reviewed-on: https://chromium-review.googlesource.com/c/1300014
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602895}