Eliminate calls to DeprecatedAtOrEmptyValue in 4 files in third_party/blink/renderer/modules/accessibility

`WTF::HashMap`'s `at()` returns an empty value when not found where
`std::map`'s `at()` throws. In order to smooth the transition,
DeprecatedAtOrEmptyValue was introduced. Now, it too must go.

Bug: 1236732
Change-Id: I79097ba3bf4a18220f699c3b5346a2f84f7bbf20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3089624
Commit-Queue: Ari Chivukula <arichiv@chromium.org>
Auto-Submit: Ari Chivukula <arichiv@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#911431}