Don't use process when not available

This breaks a compile test I am doing with the google-clousre-compiler

Author: schmidt-sebastian

packages/firestore/src/local/indexeddb_mutation_queue.ts

@@ -675,7 +675,8 @@ function convertStreamToken(token: ProtoByteString): string {
   if (token instanceof Uint8Array) {
     // TODO(b/78771403): Convert tokens to strings during deserialization
     assert(
-      process.env.USE_MOCK_PERSISTENCE === 'YES',
+      typeof process !== 'undefined' &&
+        process.env.USE_MOCK_PERSISTENCE === 'YES',
       'Persisting non-string stream tokens is only supported with mock persistence.'
     );
     return token.toString();

packages/firestore/src/local/local_serializer.ts

@@ -251,7 +251,8 @@ export class LocalSerializer {
     if (queryData.resumeToken instanceof Uint8Array) {
       // TODO(b/78771403): Convert tokens to strings during deserialization
       assert(
-        process.env.USE_MOCK_PERSISTENCE === 'YES',
+        typeof process !== 'undefined' &&
+          process.env.USE_MOCK_PERSISTENCE === 'YES',
         'Persisting non-string stream tokens is only supported with mock persistence .'
       );
       resumeToken = queryData.resumeToken.toString();

packages/firestore/src/local/simple_db.ts

@@ -159,7 +159,7 @@ export class SimpleDb {
     // If we are running in Node using the IndexedDBShim, `window` is defined,
     // but `window.navigator` is not. In this case, we support IndexedDB and
     // return `true`.
-    if (window.navigator === undefined) {
+    if (window.navigator === undefined && typeof process !== undefined) {
       return process.env.USE_MOCK_PERSISTENCE === 'YES';
     }