[webcrypto] Fix a race in BlinkPlatformImpl::crypto(), by using eager initilization of WebCryptoImpl

content/child/webcrypto/webcrypto_impl.h

Index: content/child/webcrypto/webcrypto_impl.h
diff --git a/content/child/webcrypto/webcrypto_impl.h b/content/child/webcrypto/webcrypto_impl.h
index 179de041cb5c7a1f0963ec0fb9b34a9d385674c4..485beb9698d382bb3f28da3ea4f8a39615d82c68 100644
--- a/content/child/webcrypto/webcrypto_impl.h
+++ b/content/child/webcrypto/webcrypto_impl.h
@@ -14,14 +14,19 @@
 namespace content {
 
 // Wrapper around the Blink WebCrypto asynchronous interface, which forwards to
-// the synchronous platfrom (NSS or OpenSSL) implementation.
+// the synchronous platform (NSS or OpenSSL) implementation.
 //
-// TODO(eroman): Post the synchronous work to a background thread.
+// WebCryptoImpl is threadsafe.
+//
+// EnsureInit() must be called prior to using methods on WebCryptoImpl().
 class WebCryptoImpl : public blink::WebCrypto {
  public:
   WebCryptoImpl();
+
   virtual ~WebCryptoImpl();
 
+  static void EnsureInit();
+
   virtual void encrypt(const blink::WebCryptoAlgorithm& algorithm,
                        const blink::WebCryptoKey& key,
                        const unsigned char* data,