base: Add DCHECK that double Values are finite.
base: Add DCHECK that double Values are finite.

JSON doesn't support NaN or positive/negative infinity.  I'm
hoping that this DCHECK will slightly increase the odds that
bugs are caught before they start corrupting pref files.

BUG=73856
TEST=none


Review URL: http://codereview.chromium.org/7646032

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@97111 0039d316-1c4b-4281-b951-d872f2087c98

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