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Why do this?
* It's best practice to target the latest sdk, so this makes for a
  better default.
* Many targets were neglecting to ever update this value

TBR=agrieve  # Minor BUILD.gn refactoring

Bug: None
Change-Id: Idb3983bcfb7a1ddaf13900e0acdbee425a450225
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265513
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#783112}