GitLab

o Adds Cocoa implementation of the extension installation prompt.
o Added new cross-platform implementations of the extension install error prompt.
o Got rid of unused extension install strings from early implementations.
o Added a string to display as the header of the error dialog, since it was inline English.

Patch by Andrew Bonventre <andybons@gmail.com>

BUG=19654
TEST=Install an extension on any platform, observe consistent visual behavior and messaging.
Review URL: http://codereview.chromium.org/333015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30091 0039d316-1c4b-4281-b951-d872f2087c98