Tracing screenshot size differs

I get issue to compute the speed index using speedline module. The size of the screenshots into the tracing file are not all the same. The strange thing is that the issue occurs around 66% times, running 3 times the same code, the issue occurs twice.

Steps to reproduce

• Puppeteer version: puppeteer@0.13.0
• Platform / OS version: Linux debian 3.16.0-4-amd64 #1 SMP Debian 3.16.43-2+deb8u5 (2017-09-19) x86_64 GNU/Linux

const puppeteer = require('puppeteer');
const devices = require('puppeteer/DeviceDescriptors');
const speedline = require('speedline');

async function run() {
    const iPhone = devices['iPhone 6'];
    const browser = await puppeteer.launch({args: ['--no-sandbox', '--disable-setuid-sandbox']});
    const filename = 'trace.json';

    const page = await browser.newPage();
    await page.emulate(iPhone);
    try {
        await page.tracing.start({path: filename, screenshots: true});
        await page.goto('https://www.amazon.fr');
        await page.tracing.stop();
        const results = await speedline(filename);
        console.log('Speed Index value:', results.speedIndex);
    } catch (e) {
        console.error(e);
    }
    browser.close();
}

run();

What is the expected result? Image should all be 375x667 according to the DeviceDescriptors.jsfile

{
    'name': 'iPhone 6',
    'userAgent': 'Mozilla/5.0 (iPhone; CPU iPhone OS 9_1 like Mac OS X) AppleWebKit/601.1.46 (KHTML, like Gecko) Version/9.0 Mobile/13B143 Safari/601.1',
    'viewport': {
      'width': 375,
      'height': 667,
      'deviceScaleFactor': 2,
      'isMobile': true,
      'hasTouch': true,
      'isLandscape': false
    }
  },

What happens instead? The two first pictures size are 376x667, triggering an exception Images have different sizes! into ImageSSIM

[aslushnikov]

I can repro this every other time. Seems to be a chromium bug.

[softwarerero]

I ran into the same issue today. This happens also when setting the device or viewport at all.