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The font cascade reconfiguration which was introduced as fix for AAT
shaping performance regressions in crbug.com/547912 seems to
occasionally cause CoreText crashes on OS X 10.9. We don't have a better
way of detecting this than by OS or CoreText API version number. This is
one of our top Mac crashers on Mac OS 10.9 with Chrome across versions [1].

This crash does not occur in newer versions of OS X and we can keep this
important performance optimization enabled there.

A big thanks to Robert Sesek (rsesek@) for the patient and thorough
initial investigation. Discussing and working together on this issue we
were able to identify the crash triggering code in HarfBuzz in this
case.

[1] https://bugs.chromium.org/p/chromium/issues/detail?id=576941#c74

BUG=576941, 625902

Review-Url: https://codereview.chromium.org/2173883002
Cr-Commit-Position: refs/heads/master@{#407185}