As a preparation of trusty -> xenial migration, let me specify os
dimension explicitly here.
I added os dimension for the linux builders having tests 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 v8 recipe module has default Ubuntu-14.04 dimension for linux.
https://cs.chromium.org/chromium/build/scripts/slave/recipe_modules/v8/api.py?l=428&rcl=927948978a7a101fca66f07a784aeb3ddb6650c9

Bug: chromium:954890
Change-Id: Ib2404c5aaa54d6e8e47341394d6a757a9d99874a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1578659
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60951}