Include Dataspec.httpRequestHeaders in CronetDataSource,
and OkHttpDataSource. Updated documentation of
HttpDataSource.open() to suggest that it should set request
headers (in decreasing priority) from (1) the passed DataSpec,
(2) parameters set with setRequestProperty() and (3) default
parameters set in the HttpDataSource.Factory. No mechanism
has been put in place to enforce this.
PiperOrigin-RevId: 266895574
