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utils.expose now accepts a third parameter (readonly properties).

BUG=324859

Review URL: https://codereview.chromium.org/141803016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247683 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247683