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Previous implementation did the parsing of string to base::Value
on blocking pool, and that of base::Value to specific data types
either on UI thread or on yet another post to blocking pool.

The previous implementation is slightly inefficient and moreover
involves a subtle bug 284244.

BUG=284244, 401843

This CL once landed as r288017 (Patch Set 4) and reverted.
This is the second try.

Review URL: https://codereview.chromium.org/442193002

Cr-Commit-Position: refs/heads/master@{#288275}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288275 0039d316-1c4b-4281-b951-d872f2087c98