Make imagediff build on Win64 

Just fix an implicit conversion.

BUG=166496

Review URL: https://chromiumcodereview.appspot.com/12296017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@183164 0039d316-1c4b-4281-b951-d872f2087c98
1 file changed