Server Push of tiles are not always the fastest. But, it
might be good to have the sample available to demonstrate
how Push is handled by the browser.

Change-Id: I967db9693e0889a889a36ad82248acef2c6db8d4
Reviewed-on: https://go-review.googlesource.com/38349Reviewed-by: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>