[Devtools] Renamed Network's Timeline to Waterfall

This patch renames all references of network's timeline to waterfall.
This should help with reducing the name conflict of Timeline vs
Network's timeline.

R=dgozman,lushnikov
BUG=653738

Review-Url: https://codereview.chromium.org/2483853004
Cr-Commit-Position: refs/heads/master@{#430769}