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Make OS X path "building" more buildy, less breaky

For the reasons described within the code itself, OS X can be a bit tetchy
when it comes to PKI path building. Rather than blow up in users' face with
an error, or warn them about weak signatures when strong signatures exist,
take a performance hit and do something similar to Safari, which is to
assume the OS APIs are broken/won't do the right thing, and try to fix up
the inputs prior to giving to the Security.framework.

In this case, it means trying to verify the cert as supplied (the existing
behaviour), and if that fails / gives something undesirable, begin cutting
off certs given to the OS and retrying until it gives something better or
there are no more certs to amputate.

This causes an (unmeasured) perf hit, but only for situations that would
fail for users today, or would yell at them tomorrow, so overall, it's
a worthy tradeoff.

BUG=438653, 434914, 440267
TEST= https://github.com works. https://cacert.omniroot.com works. Unit tests are happy.

Review URL: https://codereview.chromium.org/886223002

Cr-Commit-Position: refs/heads/master@{#314641}