We have been assuming in several places that ContainsDot or ToInt32 is
sufficient to check a value is a valid double or int.
Refactoring all the checks to one place and making them cope with booleans
or other unexpected types being present.

BUG=672044
R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2555323003
Cr-Commit-Position: refs/heads/master@{#41595}