Revert removal of Opera 12 code (#13395)

Revert "Final cleanup for Opera specific code from gwt-user"

This reverts commit 2d7957c969a84cab2ec6180cc53d1b3ccc656fa5.

Revert "Removes Opera from c.g.gwt.dom.*"

This reverts commit 902f6ffebd9e686731751ae962a7055af69204af.

Revert "Removes Opera from remaining c.g.gwt.user.client"

This reverts commit 2de77349c9adcb385e547c53351204579563887e.

Revert "Removes Opera from c.g.gwt.user.client.impl.DOMImpl"

This reverts commit 13bd3acb875f03bb9d1a1eee6dcf148d501800bb.

Change-Id: I84ebf9287e0f2e94a2ba4616e2ed1b71d4ec408b