Refactor how WebCrypto NSS and OpenSSL specific files are included by GN build.

content/content_child.gypi

Index: content/content_child.gypi
diff --git a/content/content_child.gypi b/content/content_child.gypi
index 1587d8e1067005d8e9d8868801a33ba26dbff780..e17f3e67a5b1c25f3cba15dad61ab37a03d7b182 100644
--- a/content/content_child.gypi
+++ b/content/content_child.gypi
@@ -200,8 +200,6 @@
       'child/webcrypto/jwk.cc',
       'child/webcrypto/jwk.h',
       'child/webcrypto/platform_crypto.h',
-      'child/webcrypto/platform_crypto_nss.cc',
-      'child/webcrypto/platform_crypto_openssl.cc',
       'child/webcrypto/shared_crypto.cc',
       'child/webcrypto/shared_crypto.h',
       'child/webcrypto/status.cc',
@@ -235,6 +233,12 @@
       'child/worker_thread_task_runner.cc',
       'child/worker_thread_task_runner.h',
     ],
+    'webcrypto_nss_sources': [
+      'child/webcrypto/platform_crypto_nss.cc',
+    ],
+    'webcrypto_openssl_sources': [
+      'child/webcrypto/platform_crypto_openssl.cc',
+    ],
   },
   'sources': [
     '<@(public_child_sources)',
@@ -289,15 +293,15 @@
       ],
     }],
     ['use_openssl==1', {
-      'sources!': [
-        'child/webcrypto/platform_crypto_nss.cc',
+      'sources': [
+        '<@(webcrypto_openssl_sources)',
       ],
       'dependencies': [
         '../third_party/openssl/openssl.gyp:openssl',
       ],
     }, {
-      'sources!': [
-        'child/webcrypto/platform_crypto_openssl.cc',
+      'sources': [
+        '<@(webcrypto_nss_sources)',
       ],
       'conditions': [
         ['os_posix == 1 and OS != "mac" and OS != "ios" and OS != "android"', {