[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
Committed:
https://crrev.com/3e197537ed66cb01d433ace38c8988e1ccb0f34d
Cr-Commit-Position: refs/heads/master@{#430769}