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This CL introduces generic functions for parsing variation parameters
into int, double, and bool (internally, variation parameters are
represented by a string).

The CL replaces some functions emulating this functionality in
ntp_snippets by the new ones in variations. More clean-up in
ntp_snippets is needed, left for further CLs.

BUG=671979

Review-Url: https://codereview.chromium.org/2558743003
Cr-Commit-Position: refs/heads/master@{#437491}