[baseview][input] Enable Baseview's OnInputEvent callback.

This is not the flip itself to Scenic input; this is the CL that enables
BaseView to respond to Scenic input.

SCN-1149 #done

SCN-938 #comment

SY-908 #comment

TEST=CQ and manual tests:
{tiles, scenic input, web_runner} - PASS
{tiles, viewmanager input, web_runner} - PASS

Change-Id: I868404ed1388caf8f2fd74ee5afa51f06b5e679d
1 file changed