Merge all KMaxRedirects=20

Too much code took upon itself to define 20 as the constant
for number of redirects. If it's going to be a constant, let it
be a single constant, in this case in the network URLRequest class.

This was noticed because the multiple kMaxRedirects in
content/browser collided in jumbo experiments.

Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ib11c1ea77ea0f1da0adb5966eb992a8f82e31177
Reviewed-on: https://chromium-review.googlesource.com/751744
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Makoto Shimazu <shimazu@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#514791}
7 files changed