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This CL changed the way how we handle conversion failures from strings.
Instead of using the default values, this CL returns 'false' to indicate
the conversion errors. This returned result is useful in case we need to
handle more complex type conversions such base::optional type.

Also, this CL simplify the type converter function definitions since we
only need to and from string conversions, not between arbitrary two
types.

BUG=938501

Change-Id: I1e6d8510c39a9a240161aa0b36c2141ab62ed0e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606772
Commit-Queue: Wei Li <weili@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659250}