Move the translate script callbacks from TranslateManager to TranslateScript

This CL refactors the way callbacks for TranslateScript are handled.
Previously, only one callback was allowed, and thus TranslateManager was
managing a global list of pending script request to be able to merge all these
requests into one callback.

As we want to move away from TranslateManager being a singleton, the management
of TranslateScript requests is moved to TranslateScript (which is also cleaner).
TranslateScript now supports multiple callbacks.

BUG=332736

Review URL: https://codereview.chromium.org/166553003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251666 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251666