Fix bug when trying to load when there are pending failures.

If you attempt to load an asset and then the first load fails, then
the second load will hang forever.  This was caused by the second load
trying to cancel the first, but the CancelableChain not handling this
case.

Closes #782

Change-Id: I79e201db44cbf47485e7221cc148bbfdde6276f7

lib/util/cancelable_chain.js

@@ -93,6 +93,11 @@ shaka.util.CancelableChain.prototype.finalize = function() {
       return Promise.resolve(data);
     }.bind(this), function(error) {
       this.complete_ = true;
+      if (this.canceled_) {
+        this.onCancelComplete_();
+        return Promise.reject(this.rejectionValue_);
+      }
+
       return Promise.reject(error);
     }.bind(this));
   }

test/util/cancelable_chain_unit.js

@@ -159,6 +159,29 @@ describe('CancelableChain', function() {
       block.resolve();
     });
 
+    it('works even if stage is rejected after being canceled', function(done) {
+      var p = new shaka.util.PublicPromise();
+      var finalComplete = false;
+      chain.then(function() {
+        return p;
+      }).finalize().then(fail).catch(function(err) {
+        finalComplete = true;
+        shaka.test.Util.expectToEqualError(cannedError, err);
+      });
+
+      chain.cancel(cannedError).catch(fail).then(function() {
+        // Delay so the catch block above can run.
+        return shaka.test.Util.delay(0.1);
+      }).then(function() {
+        expect(finalComplete).toBe(true);
+        done();
+      });
+      p.reject(new shaka.util.Error(
+          shaka.util.Error.Severity.CRITICAL,
+          shaka.util.Error.Category.MANIFEST,
+          shaka.util.Error.Code.UNABLE_TO_GUESS_MANIFEST_TYPE));
+    });
+
     it('resolves even after the finalized chain is resolved', function(done) {
       var stageComplete = false;
       var finalComplete = false;