Use GetPromisedUrlFromHeaders() for QUIC pushed streams.

Just like it is already done for HTTP/2 pushed streams, and it is
already done in QuicClientPuthPromiseIndex::Try() for QUIC pushed
streams, use GetPromisedUrlFromHeaders() instead of
GetUrlFromHeaderBlock() in QuicChromiumClientSession::HandlePromised().

Bug: 810845
Change-Id: I60a656f6e2d382bf7a1b5e288de4fb42e2a2d4db
Reviewed-on: https://chromium-review.googlesource.com/1057414
Commit-Queue: Ryan Hamilton <[email protected]>
Reviewed-by: Ryan Hamilton <[email protected]>
Cr-Commit-Position: refs/heads/master@{#558466}
3 files changed