Add missing defines = [] to boringssl/BUILD.gn

Otherwise,

[5f88f5f5...]d:\src\cr\src>out\release\gn gen out_gn
ERROR at //third_party/boringssl/BUILD.gn:58:7: Undefined variable for +=.
      defines += [ "OPENSSL_NO_ASM" ]
      ^------
I don't have something with this name in scope now.

on Windows.

TBR=brettw@chromium.org
BUG=354261

Review URL: https://codereview.chromium.org/412413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285433 0039d316-1c4b-4281-b951-d872f2087c98

third_party/boringssl/BUILD.gn

@@ -26,6 +26,7 @@ component("boringssl") {
   direct_dependent_configs = [ ":openssl_config" ]
 
   cflags = []
+  defines = []
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]