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Allow multipe set_defaults calls in GN to override one another, and allow
imported .gni files with set_defaults calls in them to not collide with
existing defaults settings as long as the settings are the same.

This will allow us to be more flexible with defaults, in particular, to
allow the defaults for templates to move to the template declaration location
rather than having to reside in BUILDCONFIG.

Add an android condition around an android-specific config in base I noticed
when testing this patch.

BUG=627978

Review-Url: https://codereview.chromium.org/2148993002
Cr-Commit-Position: refs/heads/master@{#405539}