Renaming system.gyp:ssl to system.gyp:nss and removing unused BUILD.gn

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 287 matching lines @@
 correspond to GYP ones. (This is for commonly-depended-on or weird
 targets only, don't add stuff here just because you converted it.)
 
 | *GYP*                                                                              | *GN*                                     | *Notes* (see below)  |
 |:-----------------------------------------------------------------------------------|:-----------------------------------------|:---------------------|
 | `base/base.gyp:base`                                                               | `//base`                                 |                      |
 | `base/base.gyp:base_i18n`                                                          | `//base:i18n`                            |                      |
 | `base/base.gyp:run_all_unittests`                                                  | `//base/test:run_all_unittests`          |                      |
 | `base/base.gyp:test_support_base`                                                  | `//base/test:test_support`               |                      |
 | `base/third_party/dynamic_annotations/dynamic_annotations.gyp:dynamic_annotations` | `//base/third_party/dynamic_annotations` |                      |
-| `build/linux/system.gyp:*` (except ssl)                                            | `//build/config/linux:*`                 | Linux system targets |
-| `build/linux/system.gyp:ssl`                                                       | `//crypto:platform`                      | SSL                  |
+| `build/linux/system.gyp:*` (except nss)                                            | `//build/config/linux:*`                 | Linux system targets |
+| `build/linux/system.gyp:nss`                                                       | `//crypto:platform`                      | SSL                  |
 | `net/third_party/nss/ssl.gyp:libssl`                                               | `//crypto:platform`                      | SSL                  |
 | `skia/skia.gyp:skia`                                                               | `//skia`                                 |                      |
 | `testing/gmock.gyp:gmock`                                                          | `//testing/gmock`                        | Secondary tree       |
 | `testing/gtest.gyp:gtest`                                                          | `//testing/gtest`                        | Secondary treeo      |
 | `third_party/icu/icu.gyp:icui18n`                                                  | `//third_party/icu`                      | Secondary tree, ICU  |
 | `third_party/icu/icu.gyp:icuuc`                                                    | `//third_party/icu`                      | Secondary tree, ICU  |
 | `url/url.gyp:url_lib`                                                              | `//url` || ||
 
 Notes:
 
@@ skipping 354 matching lines @@
 
 ```
 import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojo_bindings") {
   sources = [
     "foo.mojom",
   ]
 }
 ```