Cleanup decoding of flags so that invalid flags for sections other than
memory are caught correctly.

Bug: chromium:853453
Change-Id: Ia347d5f7672eee93ca3f6a743f06fba629f55cb5
Reviewed-on: https://chromium-review.googlesource.com/1104976
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Reviewed-by: Ben Smith <binji@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53972}