Currently is_linux GN variable is set to true on Chrome OS build,
but is is planned to set false. This is the preparation to
keep the compatibility.

Bug: chromium:1110266
Test: Built locally.
Change-Id: I87e8f1312c85f85a3f882698f1dce3d65e0cfab6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48348
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Hidehiko Abe <hidehiko@google.com>