Fix error when VitalSource integration injects client into content frame

Construction of the integration was recently moved to happen earlier in the
Guest.  This caused a regression in the VitalSource integration, which may
synchronously call `Guest#injectClient` when constructed. `injectClient` in turn
relies on `_hypothesisInjector` being initialized, which was not the case.

The error could be seen in the dev server's VS test case at
http://localhost:3000/document/vitalsource-epub.

Fix the issue by moving `_hypothesisInjector` initialization to happen earlier
in the Guest constructor, before `createIntegration` is called.

src/annotator/guest.js

@@ -176,6 +176,10 @@ export default class Guest {
       source: 'guest',
     });
 
+    // Set up automatic and integration-triggered injection of client into
+    // iframes in this frame.
+    this._hypothesisInjector = new HypothesisInjector(this.element, config);
+
     /**
      * Integration that handles document-type specific functionality in the
      * guest.
@@ -198,10 +202,6 @@ export default class Guest {
     this._sidebarRPC = new PortRPC();
     this._connectSidebar();
 
-    // Set up automatic and integration-triggered injection of client into
-    // iframes in this frame.
-    this._hypothesisInjector = new HypothesisInjector(this.element, config);
-
     this._bucketBarClient =
       this._frameIdentifier === null
         ? new BucketBarClient({

src/annotator/test/guest-test.js

@@ -37,6 +37,7 @@ describe('Guest', () => {
 
   let FakeBucketBarClient;
   let fakeBucketBarClient;
+  let fakeCreateIntegration;
   let FakePortRPC;
   let fakePortRPCs;
   let fakeIntegration;
@@ -134,6 +135,8 @@ describe('Guest', () => {
       uri: sinon.stub().resolves('https://example.com/test.pdf'),
     };
 
+    fakeCreateIntegration = sinon.stub().returns(fakeIntegration);
+
     fakeHypothesisInjector = {
       destroy: sinon.stub(),
       injectClient: sinon.stub().resolves(),
@@ -165,7 +168,7 @@ describe('Guest', () => {
         BucketBarClient: FakeBucketBarClient,
       },
       './integrations': {
-        createIntegration: sinon.stub().returns(fakeIntegration),
+        createIntegration: fakeCreateIntegration,
       },
       './highlighter': highlighter,
       './hypothesis-injector': { HypothesisInjector: FakeHypothesisInjector },
@@ -1330,5 +1333,21 @@ describe('Guest', () => {
       assert.calledWith(FakeHypothesisInjector, guest.element, config);
       assert.calledWith(fakeHypothesisInjector.injectClient, frame);
     });
+
+    it('can be called by the integration when created', async () => {
+      const config = {};
+      const frame = {};
+
+      // Simulate the integration injecting the client into pre-existing content
+      // frames when created. The VitalSource integration does this for example.
+      fakeCreateIntegration.callsFake(annotator => {
+        annotator.injectClient(frame);
+        return fakeIntegration;
+      });
+      const guest = createGuest({});
+
+      assert.calledWith(FakeHypothesisInjector, guest.element, config);
+      assert.calledWith(fakeHypothesisInjector.injectClient, frame);
+    });
   });
 });