In PolyGerrit, context.call() should will now use same context URL as in
GWT UI.
For change context:
/changes/project~changeid/pluginname~action
For revision context:
/changes/project~changeid/revisions/revisionid/pluginname~action
Feature: Issue 7685
Change-Id: I6db97ecdec9ab1f923148e2b65c8c347201f7ee3