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There was an optimization in preview_generator.js that didn't request a new preview in certain cases.  This CL relaxes the checks so that changing to a new printer always requests a new preview.

In the case where the preview really DOESN'T need to be regenerated an optimization on the C++ side will prevent unneeded regenerations.

BUG=140826


Review URL: https://chromiumcodereview.appspot.com/10826214

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@150699 0039d316-1c4b-4281-b951-d872f2087c98