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Also, add a browser test for that, and add GYP rules for the downloads page browser tests.

BUG=264184

Review URL: https://chromiumcodereview.appspot.com/23851007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222054 0039d316-1c4b-4281-b951-d872f2087c98