Use the list of keywords to tighten the CannotBeKeyword scan flag to
also exclude lower case letters which are not present in any of the
keywords.

Change-Id: I6a00b5f5ee8f47088539806f15890a7489441fea
Reviewed-on: https://chromium-review.googlesource.com/c/1347475
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57754}