Cleanup CanDownload WebContentsDelegate API

It seems we were passing an opaque request ID in WebContentsDelegate::CanDownload
that wasn't used anywhere. Instead, for <webview>, we would simply like the URL
that generated the download request. This CL simplifies the API and reduces cruft
and code complexity

BUG=364141, 330264
TBR=sgurun@chromium.org

Review URL: https://codereview.chromium.org/286543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270242 0039d316-1c4b-4281-b951-d872f2087c98
14 files changed