Update 'mb' setting for Linux ARM bots
Update 'mb' setting for Linux ARM bots

- Add 'swarming' settings since these bots now use swarming.
- Remove 'arm_linux_tester' and corresponding 'none' settings.
- Remove now unused 'arm_float' settings.

CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_arm_compile
BUG=159519

Review URL: https://codereview.chromium.org/1470623002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#361028}

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