Allow page to suppress all of its content from search.

To suppress things from search, we usually use preferencecontroller to
control individual items. But a page can sometimes to disabled
entirely. Now it can set isPageSearchEnabled to false to achieve that.

Change-Id: I39424b7fdc6460e27fd7c83cf09b5ace12d1cc37
Fix: 36201167
Test: make RunSettingsRoboTests