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Only enables the warning for chromium_code.n

Also fix the last few -Wextra-semi instances in sanitizer and linux/ozone builds.

Getting to this state required fixing > 3000 unique warnings in 15+ different
repositories.  If this breaks some internal-only build somewhere, either consider
using no_chromium_code for your internal code, or fix the warnings -- it's
pretty easy. (Build with treat_warnings_as_errors=false, then open the error log
in vim, run `:%g/warning:/t$` to copy the warnings to the end of the buffer,
remove all but just those lines, run `:sort u` to get uniques, then save as
`tmp.txt` and run `:cf %` and then use something like `:map m :w<cr>:cn<cr>`
to go through them quickly.)

TBR=rsesek

Bug: 926235
Change-Id: Ica629737523ff6bbd756edc44d182f35ff6cf8ac
Reviewed-on: https://chromium-review.googlesource.com/c/1483396Reviewed-by: Scott Violet <sky@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634826}