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This change fixes an issue inside IsContiguousIterator that would fail
compilation if it was instantiated with an iterator to a container that
has a non-trivial value_type.

This was because `IsContiguousIterator<It>` instantiated
`std::basic_string<iter_value_t<It>>` to determine whether `It` was an
iterator for a `std::basic_string`. This would then trigger a
static_assert inside basic_string if `iter_value_t<It>` was not a trivial
type. Now IsContiguousIterator<It> only attempts to instantiate a
std::basic_string if `iter_value_t<It>` is a trivial type, avoiding this
issue.

Bug: 828324
Change-Id: I1ab8d10b1847cca09041cc3f2fe623d2a04602fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2632958
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844711}