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This specifies the viewer that will be used to render documents that are
a supported MIME type for the extension.

BUG=366409

Review URL: https://codereview.chromium.org/254503002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266821 0039d316-1c4b-4281-b951-d872f2087c98