Implement new dangerous download reporting dialog for UNCOMMON_DOWNLOAD, in Views

Adds a new dialog, DownloadFeedbackDialogView, to ask people whether they want to participate in the upload program. Currently only hooked up to UNCOMMON_DOWNLOAD.

Still TODO:
- need to add dialog to DANGEROUS_HOST as well
- need to add new histogram for dialog once DANGEROUS_HOST is in place

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How to test [Win & CrOS only]:
1. Visit http://testsafebrowsing.appspot.com/, click on #5, then click discard, then click any choice in the dialog
2. If you repeat #1, you shouldn't see the dialog again unless you delete the preferences file in the User Data directory
3. If you do #1 in two separate windows, only one of them should trigger the dialog

BUG=312533
TBR=asanka@chromium.org

Review URL: https://codereview.chromium.org/147593002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249796 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249796