This feature will be controlled by the caller.
Cronet calls the register/unregisterQuicConnectionClosePayload APIs only
when the Chromium experiment flag is enabled
(http://crrev.com/c/6421621/).
This CL also updates testRegisterQuicConnectionClosePayload
to be skipped on U or older releases since FIREWALL_CHAIN_BACKGROUND is
supported on V or higher releases.
Flag: EXEMPT guarded by chromium flag
Bug: 311792075
Test: TH
Change-Id: Ib008e63c585cfe8775b25783cd55b0873bb5e2f0
2ea5939672