Position dialogs using absolute rather than fixed.
This allows them to be scrolled into view if the window is too small.
Note that the linked bug actually asks for the dialogs to be centered.
I haven't done that because the rest of the HTML content is not centered,
and having the dialog appear at a different location relative to the
content, depending on the window size doesn't seem right. This approach
improves accessibility without changing the UX for most users.
BUG=245395
NOTRY=true
Review URL: https://chromiumcodereview.appspot.com/
16382002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@204120 0039d316-1c4b-4281-b951-d872f2087c98