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clang will now warn on non-empty case and default blocks that don't end
with a break statement.  For cases where fallthrough is intentional,
end the block with FALLTHROUGH; from base/compiler-specific

Depends on these changes to DEPS rolling into chromium:
https://pdfium-review.googlesource.com/#/c/pdfium/+/24790
https://chromium-review.googlesource.com/c/angle/angle/+/895728
https://chromium-review.googlesource.com/c/v8/v8/+/895704
https://chromium-review.googlesource.com/c/breakpad/breakpad/+/895705
https://github.com/google/cld3/pull/9

We'll enable this for more build configurations once they can build
with the warning enabled.

(Also change a single file that was needed to get the dcheck_always_on
build to work, and another one that is built only in static builds.)

Bug: 177475,808047
Change-Id: Ic6a3fbf78a9cb2b5e928ce7973e67e4a39629dfd
Reviewed-on: https://chromium-review.googlesource.com/895726Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533729}