[xhrio] Support returning headers starting from `ReadyState.LOADED` (i.e. `HEADERS_RECEIVED`) state. RELNOTES: n/a
[xhrio] Support returning headers starting from `ReadyState.LOADED` (i.e. `HEADERS_RECEIVED`) state.
RELNOTES: n/a

PiperOrigin-RevId: 441102565
Change-Id: I6d30f4f668fff3f31708ac6da25b3fc3c1d869cb

google/closure-library,google/closure-library,google/closure-library,google/closure-library,google/closure-library