gtkui: Improve BuildXkbStateFromGdkEvent

- Add DCHECKs to ensure consistency of both GdkEventKey's group and
state fields, before translating it into Xkb's state composed value
- Fix comment documenting BuildXkbStateFromGdkEvent function
- Reference official Xkb docs instead of the previous one

Bug: None
Change-Id: I9e2c4f9db20bb231268765a5fd71ca7ceb4f3f55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902435
Commit-Queue: Nick Yamane <nickdiego@igalia.com>
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713599}
1 file changed