DevTools: remove Bindings.NetworkMapping.addMapping/removeMapping methods
This patch removes fileSystemMapping from Bindings.NetworkMapping. As a
result, the fileSystemMapping starts to listen to FileSystemAdded/FileSystemRemoved
events by itself.
BUG=
649837
R=dgozman
Committed:
https://crrev.com/c7500edf7716fe6af09e4462edbe8dfc7c1e524b
Cr-Commit-Position: refs/heads/master@{#432252}