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Enables the developer options on <= Dev builds by default and adds a way
to activate them on Beta/Stable channels via 7 taps on the chrome
version in the preferences, in android-like style.

Bug: 898512
Change-Id: Ife44df843e45d660485fb464209964c957a9c887
Reviewed-on: https://chromium-review.googlesource.com/c/1349309
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613034}