Fix a bunch of generated file build flakes in //extensions

extensions/common/api/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/json_schema_api.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 
 assert(enable_extensions)
 
@@ skipping 94 matching lines @@
 
 json_schema_api("api_registration") {
   sources = schema_files
   impl_dir = "//extensions/browser/api"
   bundle_registration = true
   bundle_name = ""
 
   deps = [
     ":api",
     "//device/serial",
+    "//extensions/browser/api",

[lazyboy, 2016/10/26 17:22:23]

Not sure if you want this, over to Devlin for reviewing.

[Devlin, 2016/10/26 19:30:28]

Hmm....  So I'm assuming we need this because the generated files #include all
the browser apis in order to register the functions.  Ideally, this would just
go in extensions/browser/api, since the output file also goes there, but then we
can't use the variables here (like schema_files) as easily.  Dirk might know if
there's an easy way for this to expose schema_files as a variable, which could
then be included from the extensions/browser/api BUILD.gn, and if that's a
reasonable approach to take here.

[Devlin, 2016/10/26 19:31:00]

(fwiw, I think this actually works, but it's a little messy to reference
"browser" from a common at all - even in a BUILD.gn file)

[tsniatowski, 2016/10/26 20:43:42]

Yes, what ends up happening is that the build of
obj/extensions/common/api/api_registration/generated_api_registration.o includes
gen/content/public/common/window_container_type.mojom.h and friends via one of
the api headers.

FWIW, the dependency / reference is already there, this CL only makes it
explicitly listed (so the build is not flaky). Seemed better than adding a
direct //content dep just because it happens to fix the flake.

[Dirk Pranke, 2016/10/26 21:49:21]

The way to share variables between files is to move the variable into a .gni
file; that gni file can then be imported by anything that needs it. In this
particular case, that looks easy enough.

[Devlin, 2016/10/26 21:57:52]

tsniatowski@, can we try doing this and moving this action to
extensions/browser/api/BUILD.gn?  I think that would be a lot cleaner.

(I realize this is only making the BUILD.gn reflect reality, but it's a good
opportunity to make it better. :))

[tsniatowski, 2016/10/27 08:18:58]

OK, I'll have a look. For consistency, do you want an equivalent change in
//extensions/shell: extract schema args to .gni, move api registration to
browser/?

     "//extensions/common/api/cast_channel:cast_channel_proto",
     "//skia",
   ]
 
   # Inherit uncompiled_sources, uncompiled_bundle_schema_sources, and
   # root_namespace.
 }