Add Views-specific download danger prompt dialog

Create a Views web contents modal dialog implementation of the download
danger prompt dialog, so that we can apply bold text formatting to the
lead text in the dialog. Adds a TODO for the same change on other platforms.

See the associated bug for a screenshot.

BUG=305387

Review URL: https://codereview.chromium.org/28673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230308 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230308
Constrains keyboard context menu origin to the visible bounds of view

Without this we may try to show context menu outside the visible
bounds, which looks odd.

BUG=308886
TEST=see bug
R=ben@chromium.org

Review URL: https://codereview.chromium.org/35153008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230309 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230309