fix: some websites using WebComponents V0 not loading

[codebytere]

Description of Change

Closes #21596.
Refs: https://chromium-review.googlesource.com/c/chromium/src/+/1869562

Fixes an issue where any website using Polymer 2.x would not load, as Polymer 2.x still uses HTML Imports as a loading mechanism. This adds the flag to enable them back for the time being, since Electron does not support origin trials and so wouldn't respect individual sites enabling them.

Another potential approach to this would be to add HTMLImports to the enableBlinkFeatures defaults, since that's more granular. Finally, we may choose to simply force consumers to work around the issue themselves, but I would say that for now I personally am in favor of leaving this enabled while it affects a nontrivial number of sites.

cc @nornagon @deepak1556 @zcbenz

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed an issue where sites using ShadowDOMV0, CustomElementsV0, or HTMLImports would not load properly.

[deepak1556]

I think build flag is overkill for this one, apps that don't want the feature can just pass it through --disable-features to disable it.

LGTM otherwise, nice find. I am surprised youtube is an affected site based on the description from the CL.

[zcbenz]

I'm good with this, but all Windows PRs are failing. They seem all flaky, we should have a retry of the tests before merging.

[release-clerk]

Release Notes Persisted

Fixed an issue where sites using ShadowDOMV0, CustomElementsV0, or HTMLImports would not load properly.

[trop]

I have automatically backported this PR to "8-x-y", please check out #21866

[nornagon]

Hm, this shouldn't have been merged as new-pr was still applied

[zcbenz]

Sorry for merging this too quickly, I didn't know the meaning of new-pr label.