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This policy is more in accord with the design doc changes. Moves to
showing the dialog upon DOMContentLoaded in source pages, and then
subsequently for user gestures.

R=leng@chromium.org
BUG=332115,364159

Review URL: https://codereview.chromium.org/243543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266855 0039d316-1c4b-4281-b951-d872f2087c98