Introduce TranslateService and TranslateDownloadManager

This CL removes the TranslateDelegate interface and introduces
TranslateService and TranslateDownloadManager.

This will enable (in a future CL) to move TranslateScript and
TranslateLanguageList in the Translate component, and
TranslateDownloadManager will own them.

BUG=332736

Review URL: https://codereview.chromium.org/145023015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247724 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247724
Fix nit introduced in r247511

BUG=334897
TBR=rvargas@chromium.org

Review URL: https://codereview.chromium.org/149513004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247725 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247725