GN: default to -O1

In a GN world we are no longer tied to the Debug/Release dichotomy of
GYP, and extra_cflags makes it trivially easy to override any flags we
set by default.  These two together mean we don't have to make the default
Debug build pointlessly unoptimized.

-O1 takes negligibly longer to compile than -O0, doesn't make debugging
difficult, and the code is considerably better.  DM runs ~4x faster this way.

Backstory: I just re-noticed we do this for our Debug MSAN builds, which has
had a serious wall-time slimming effect.  I was about to restore it to my GN
MSAN builds when I realized, hey, maybe we should do this for all builds.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2322753002

Review-Url: https://codereview.chromium.org/2322753002

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