Get progress events from Fetch

[joeyparrish]

Split off from #1051:

We need progress events from Fetch so that after #1051, we don't have to prefer XHR again.

[rounce]

I did it this way:

let fetch = shaka.net.HttpFetchPlugin.fetch_;
  let promise = fetch(uri, init).then(function(response) {
    clearTimeout(timeout);
  
    let headers = {};
    /** @type {Headers} */
    let responseHeaders = response.headers;
    responseHeaders.forEach(function(value, key) {
      // Since IE/Edge incorrectly return the header with a leading new line
      // character ('\n'), we trim the header here.
      headers[key.trim()] = value;
    });
      
    const reader = response.body.getReader();
    let loaded = 0;
    return new ReadableStream({
      start(controller) {
        reader.read().then(function processData(result) {
          if (result.done) {
            controller.close();
            return;
          }
          controller.enqueue(result.value);
          loaded += result.value.byteLength;
          // progress = loaded / sizeTotal
          return reader.read().then(processData);
        });
      }
    });
  })
  .then((stream) => new Response(stream))
  .then((response) => {
    return response.arrayBuffer().then(function(arrayBuffer) {
      return shaka.net.HttpPluginUtils.makeResponse(headers,
          arrayBuffer, response.status, uri, response.url, requestType);
    });
  })
  .catch(function(error) {
...

[joeyparrish]

Released in v2.5.0-beta2.