(core) Broadcast doc usage updates to clients

Summary:
Introduces a new message type, docUsage, that's broadcast to all connected
clients whenever document usage is updated in ActiveDoc.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3451

app/client/ui/setupPage.ts

@@ -1,10 +1,14 @@
+import {get as getBrowserGlobals} from 'app/client/lib/browserGlobals';
 import {AppModel, TopAppModelImpl} from 'app/client/models/AppModel';
 import {setUpErrorHandling} from 'app/client/models/errors';
 import {buildSnackbarDom} from 'app/client/ui/NotifyUI';
 import {addViewportTag} from 'app/client/ui/viewport';
 import {attachCssRootVars} from 'app/client/ui2018/cssVars';
+import {BaseAPI} from 'app/common/BaseAPI';
 import {dom, DomContents} from 'grainjs';
 
+const G = getBrowserGlobals('document', 'window');
+
 /**
  * Sets up error handling and global styles, and replaces the DOM body with
  * the result of calling `buildPage`.
@@ -14,6 +18,12 @@ export function setupPage(buildPage: (appModel: AppModel) => DomContents) {
   const topAppModel = TopAppModelImpl.create(null, {});
   attachCssRootVars(topAppModel.productFlavor);
   addViewportTag();
+
+  // Add globals needed by test utils.
+  G.window.gristApp = {
+    testNumPendingApiRequests: () => BaseAPI.numPendingRequests(),
+  };
+
   dom.update(document.body, dom.maybe(topAppModel.appObs, (appModel) => [
     buildPage(appModel),
     buildSnackbarDom(appModel.notifier, appModel),