Cheat to go fast, but be careful in debug mode.

Turns out recording actual text metrics is currently really slow.
If we want to compete with the (cheating) status quo, gotta cheat too.

BUG=skia:
R=reed@google.com, robertphillips@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/494483002
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