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This change cleans up a bunch of things now that we don't need to
support GYP:

'gn' becomes the default build type, so we no longer need 'gn_' in
the config name or need to specify the 'gn' mixin.

'swarming' and 'noswarming' are no longer needed in the config
names since the swarming flag was only needed for GYP.
'archive_gpu_tests' is also gone, for the same reason.

The 'none' builder type is gone; builders that don't build simply
aren't listed.

The 'configs' and 'mixins' sections are sorted and de-duped.

Any non-clang win builder that was specifying both goma and
minimal_symbols is now just goma, since goma + win + !clang
implies minimal symbols. A follow-on CL should make goma
imply minimal symbols across the board ...

'x64' no longer needs to be specified explicitly on windows
since it is the default.

The 80-col line limit in mb_config.pyl was dropped so that
configs can be listed on one line.

The 'symbolized' config is being removed; it was never
implemented in GN since we dropped those builders;
the one remaining v8 builder has been reporting
warnings since :).

R=thakis@chromium.org, machenbach@chromium.org
BUG=634184

Review-Url: https://codereview.chromium.org/2357483002
Cr-Commit-Position: refs/heads/master@{#420529}