Tweak "Typical sources and deps modifications" section.

tools/gn/docs/cookbook.md

 # GYP->GN Conversion Cookbook
 
 [TOC]
 
 ## Targets
 
 | *GYP*                                            | *GN*                                               |
 |:-------------------------------------------------|:---------------------------------------------------|
 | `'type': 'static_library', 'name': 'foo',`       | `static_library("foo") {` or `source_set("foo") {` |
 | `'type': 'shared_library', 'name': 'foo',`       | `shared_library("foo") {`                          |
@@ skipping 104 matching lines @@
 | `'conditions': [['chromeos==1', {`   | `if (is_chromeos) {` |
 
 ## Typical sources and deps modifications
 
 ### GYP
 
 ```
 'sources': [
   'a.cc',
   'b.cc',
+  'c.cc',
 ],
 'dependencies': [
   '<(DEPTH)/base/base.gyp:foo',
 ],
 'conditions': [
   ['OS=="win"': {
     'sources!': [
        'a.cc',
     ],
     'sources': [
       'foo.cc',
     ],
     'dependencies': [
       '<(DEPTH)/base/base.gyp:bar',
     ],
   }, {
     'sources/': [
       ['exclude', '^b\\.cc$'],
     ],
   }],
 ],
 ```
 
 ### GN
 
 ```
 sources = [
-  "a.cc",
-  "b.cc",
+  "c.cc",
 ]
 deps = [
   "//base:foo",
 ]
 
 if (is_win) {
-  sources -= [ "a.cc" ]
-  sources += [ "foo.cc" ]
+  sources += [
+    "b.cc",
+    "foo.cc',
+  ]
   deps += [ "//base:bar" ]
 } else {
-  sources -= [ "b.cc" ]
+  sources += [ "a.cc" ]
 }
 ```
 
+Note that in GN we prefer to only add files when needed, and don't add all of
+them at first only to remove them later like in gyp.
+
 ## Variable mappings
 
 ### Build configuration
 
 Build configuration and feature flags are usually global in GYP. In GN
 we try to limit global variables and instead put variables used by only
 some files into `.gni` files. These files are then imported into your
 buildfile by specifying at the top:
 
 ```
@@ skipping 491 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```