specify os dimension explicitly for test triggered from linux builder

As a preparation of trusty -> xenial migration, let me specify os
dimension explicitly here.
I added 'linux-trusty' mixins for the builders having linux in its name
and test_suites property.

I will change this dimension gradually with gradual swarming test bots
trusty -> xenial migration.

Currently this should not change ubuntu's version of bots running tests
because chromium_swarming recipe module has Ubuntu-14.04 dimension for linux.
https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/chromium_swarming/api.py?l=460&rcl=927948978a7a101fca66f07a784aeb3ddb6650c9

Bug: 954450
Change-Id: I6f16dbbe2e52ac79a05a52bfb30cdd2ece0462a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578722
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653449}
12 files changed