UserAgent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:49.0) Gecko/20100101 Firefox/49.0

Steps to reproduce the problem:
Simple PoC (this uses the toolchain in //tools/gn/example):

config("precompiled_headers") {
  precompiled_header = "build/precompile.h"
  precompiled_source = "//build/precompile.h"
}

executable("hello") {
  sources = [
    "hello.cc",
  ]

  cflags_cc = [ "-std=gnu++14" ]

  # If the following line is uncommented, the generated
  # hello.ninja's cflags_cc variable will be empty (instead of -std=gnu++14).
  configs += [ ":precompiled_headers" ]
}

What is the expected behavior?
Using a precompiled header, even though the toolchain doesn't support it, should lead to an error.

What went wrong?
If a target tries to use precompiled headers, but the toolchain's tool() for one of the used languages doesn't specify a precompiled_header_type,
the flags variable for that language (e.g. cflags_cc) will always be empty.

E.g.: The cxx tool has no precompiled_header_type variable -> Targets with C++ sources that use precompiled headers will always get an empty cflags_cc variable in the Ninja file.

Did this work before? No 

Chrome version: <Copy from: 'about:version'>  Channel: n/a
OS Version: 6.1 (Windows 7, Windows Server 2008 R2)
Flash Version: Shockwave Flash 23.0 r0