Add renderStrict that returns the same SanitziedContent as the input.

R=xtof
DELTA=61 (60 added, 0 deleted, 1 changed)


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git-svn-id: http://closure-library.googlecode.com/svn/trunk@2455 0b95b8e8-c90f-11de-9d4f-f947ee5921c8