Use depfiles for mojom generation.
Currently, if foo.mojom imports bar.mojom and bar.mojom changes,
foo.mojom will not be regenerated. If the change to bar.mojom changes
which traits headers should be included by foo.mojom.cc by adding or
removing a typemapped type, this can cause compile failures. This CL
fixes this by generating a depfile for each mojom generation step so
changing any mojoms imported by a particular mojom cause the bindings
for that mojom to be regenerated.