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

content/child/BUILD.gn

Index: content/child/BUILD.gn
diff --git a/content/child/BUILD.gn b/content/child/BUILD.gn
index 07f92387535456f012f9eca4806b3cb8bf0732e7..873c13b8779b42c1cc98df112a291b0115ffbca0 100644
--- a/content/child/BUILD.gn
+++ b/content/child/BUILD.gn
@@ -107,8 +107,10 @@ source_set("child") {
   }
 
   if (!use_openssl) {
-    sources -= [ "webcrypto/platform_crypto_openssl.cc" ]
+    sources += rebase_path(content_child_gypi_values.webcrypto_nss_sources,
+                           ".", "//content")
   } else {
-    sources -= [ "webcrypto/platform_crypto_nss.cc" ]
+    sources += rebase_path(content_child_gypi_values.webcrypto_openssl_sources,
+                           ".", "//content")
   }
 }