Remove HTTP/2 draft-15 enum values and test cases.
Remove NextProto::kProtoSPDY4_15 and AlternateProtocol::NPN_SPDY_4_15 enum
values. Remove HTTP/2 draft-15 test cases. (Our draft-15 implementation is
currently identical to draft-14 anyway.) Leave
ConnectionInfo::CONNECTION_INFO_HTTP2_15 enum value as is might be persisted to
cache.
Note that "h2-15" has not been advertised by Chrome since
https://crrev.com/
822153003, so these enum values were not normally used.
This CL is git cl formatted.
BUG=447587
Review URL: https://codereview.chromium.org/
993033002
Cr-Commit-Position: refs/heads/master@{#319973}