Flip the main linux x64 bots from GYP to GN.
Flip the main linux x64 bots from GYP to GN.

This CL flips the following bots to build w/ GYP instead of GN by default:

- tryserver.chromium.linux:
  - linux_chromium_rel_ng
- chromium.linux:
  - Linux Builder
  - Linux Tester

R=brettw@chromium.org
BUG=432959

Review URL: https://codereview.chromium.org/1323353002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#347431}

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