Lazy Images: Fix conflict with photon after switching to use srcset

[ebinnion]

Some JavaScript that sets height and width for Photon images had some checks for data-lazy-src that would postpone adding those height and width details. Here's an example:

jetpack/modules/photon/photon.js

Line 15 in dc3e81b

if ( $this.data( 'lazy-src' ) && $this.attr( 'src' ) !== $this.data( 'lazy-src' ) ) {

After switching to use srcset instead of src for lazy loading, and removing data-lazy-src, the height and width attributes were being set prematurely, while the image was still 1x1. 😞

Changes proposed in this Pull Request:

Add data-lazy-src back so that the photon image size checks are postponed until the image is actually loaded.

Ideally, we should probably make changes directly to the photon.js and devicepx.js files. But, I'd rather postpone that until we verify that srcset will work in the wild as expected.

Testing instructions:

On a connected site:

• Enable photon and lazy images
• Create a post and insert an image
• View source of post and remove any height and width attributes that were added
• Load post
• Ensure that image does load but is 1x1 😞
• Checkout this branch
• Load post again
• Ensure image loads at proper size

Proposed changelog entry for your changes:

None

[jetpackbot]

That's a great PR description, thank you so much for your effort!

Generated by 🚫 dangerJS

[oskosk]

Tests great. Confirmed that fixes the issue