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There are two main ctors for URLPattern
- one takes valid schemes and is used with the Parse() method (which
  handles failure)
- one that takes schemes and a string pattern, and assumes parsing must
  succeed

The latter currently has a NOTREACHED() and an error log. Upgrade this
to a DCHECK, since it should never, ever be hit (especially following
the identifying case for crbug.com/856948).

This should - hopefully - help ensure that no new cases are introduced,
and flush out any other cases that may exist.

Bug: 856948
Change-Id: I1982d10e92c72d1d8ca6da03644072a73a0c7580
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450707Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815870}