crypto/rand, crypto/x509: add js/wasm architecture

This commit adds the js/wasm architecture to the crypto packages.

Updates #18892

Change-Id: Id41a9d54920746d5019cbeedcff1b83874f2ef73
Reviewed-on: https://go-review.googlesource.com/110095
Reviewed-by: Austin Clements <austin@google.com>

src/crypto/rand/rand.go

@@ -15,6 +15,7 @@ import "io"
 // On OpenBSD, Reader uses getentropy(2).
 // On other Unix-like systems, Reader reads from /dev/urandom.
 // On Windows systems, Reader uses the CryptGenRandom API.
+// On Wasm, Reader uses the Web Crypto API.
 var Reader io.Reader
 
 // Read is a helper function that calls Reader.Read using io.ReadFull.

src/crypto/rand/rand_js.go

@@ -0,0 +1,25 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm
+
+package rand
+
+import "syscall/js"
+
+func init() {
+	Reader = &reader{}
+}
+
+var jsCrypto = js.Global.Get("crypto")
+
+// reader implements a pseudorandom generator
+// using JavaScript crypto.getRandomValues method.
+// See https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues.
+type reader struct{}
+
+func (r *reader) Read(b []byte) (int, error) {
+	jsCrypto.Call("getRandomValues", b)
+	return len(b), nil
+}

src/crypto/x509/root_js.go

@@ -0,0 +1,10 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm
+
+package x509
+
+// Possible certificate files; stop after finding one.
+var certFiles = []string{}

src/crypto/x509/root_unix.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build dragonfly freebsd linux nacl netbsd openbsd solaris
+// +build dragonfly freebsd js,wasm linux nacl netbsd openbsd solaris
 
 package x509