[BUG] [Chromium/WebKit] Request object does not contain postData for file/blob

Context:

System:

• OS: Windows 10 10.0.19043

Binaries:

• Node: 12.13.1 - C:\Program Files\nodejs\node.EXE
• Yarn: 1.19.1 - ~\AppData\Roaming\npm\yarn.CMD
• npm: 6.14.2 - C:\Program Files\nodejs\npm.CMD

Languages:

• Bash: 4.4.20 - C:\WINDOWS\system32\bash.EXE

npmPackages:

• playwright: ^1.10.0 => 1.10.0 / tip-of-tree

Code Snippet

Help us help you! Put down a short code snippet that illustrates your bug and
that we can run and debug locally. For example:

const playwright = require('.');

(async () => {
  const browser = await playwright.webkit.launch();
  const page = await browser.newPage();

  await page.goto('http://example.com');

  const requestPromise = page.waitForRequest(() => true);
  const [request] = await Promise.all([
    page.waitForRequest("**/*"),
    page.evaluate(() => {
      var file = new File(['file-contents'], 'filename.txt');

      fetch('/data', {
        method: 'POST',
        headers: {
          'content-type': 'application/octet-stream'
        },
        body: file
      });
    })
  ])
  console.info('request.method()', request.method());
  console.info('request.url()', request.url());
  console.info('request.headers()', request.headers());
  console.info('request.postData()', request.postData());
})();

Describe the bug

the request.postData() contains null in Chromium and WebKit. In Firefox it's filled.