Always produce an optimized library on Chromium

Always produce an optimized library on Chromium

The SwiftShader library, when used to render Layout Tests in Debug,
causes too many timeouts. For that reason, we need the Debug version
of the library have better overall performance. In Debug, the
"default_optimization" defaults to "no_optimization". See:
https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn?l=1698
In Release, "default_optimization" defaults to "optimize", so in
order to at least use the same optimization flags in both case, we
replace "default_optimization" by "optimize" in Debug.

The Win 7 (dbg) bot ran successfully with this change (the
telemetry_tests were failing when building the full Release version
of the SwiftShader library with the Debug version of Chromium).

Change-Id: I3529160b539319e26f9898d2588536d378c3ee01
Reviewed-on: https://swiftshader-review.googlesource.com/11469
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>