linux: prompt before overwriting a file in downloads
linux: prompt before overwriting a file in downloads

We rename implicit downloads so they don't conflict with existing
files, but we should prompt if you manually type in an existing
filename.

BUG=26117

Review URL: http://codereview.chromium.org/537005

git-svn-id: http://src.chromium.org/svn/trunk/src@35859 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

Former-commit-id: 8ce11ccd7e6ffed5e6b8df3063a7fc28083cc3c1
meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser,meego-tablet-ux/meego-app-browser