Add missing inline keyword for various constants in //chrome headers

Follow guidance on https://groups.google.com/a/chromium.org/g/chromium-dev/c/OXxJy5L96jI/m/tRZHIeZwAgAJ
and add the missing inline keyword so they only have one definition.

Also rewrite: `const char* const kFoo = "foo";`
to: `inline constexpr char kFoo[] = "foo";`

Also flip `constexpr inline` to `inline constexpr` for consistency.

Change-Id: I4b9ff27c78faa6ff8236060df57534bf8e6c8a18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6527888
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458156}