[webview_flutter_web] Add WebWebViewController.runJavaScript

[sophiebremer]

This PR (replaces #9835) adds an implementation for WebWebViewController.runJavaScript and changes the way WebWebViewController.loadHtmlString injects HTML code while also adding support for the baseUrl option.

The following issues are fixed:

• [webview_flutter] Support runJavaScript for Web when the content has the same origin flutter#173899

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[sophiebremer]

Hello @mdebbar!

Please provide guidance. Not all tests are successful, and how to add a useful test for WebWebViewController.runJavaScript without browser context is unclear to me.

[stuartmorgan-g]

team-web will still need to do a full review, but in the meantime: won't this change to loadHtmlString silently break any case where baseUrl is passed, as a regression from the previous behavior?

[sophiebremer]

This is valid concern. I see two different options to address it:

1. The previous implementation gets added as a fallback in the catch block.

2. We introduce a new controller parameter option to activate this PRs implementation and use the existing implementation by default as before.

What are your suggestions / thoughts?

[stuartmorgan-g]

@mdebbar How do you want to handle branching for different origin setups?

2. We introduce a new controller parameter option to activate this PRs implementation and use the existing implementation by default as before.

Would the parameter add any information we don't already have? Can't we determine at runtime if the origins match?

[sophiebremer]

Hello! We realised that we need more CORS flexibility which is in conflict with backward compatibility. Therefore we decided to go with a custom implementation based on the webview interfaces to support our usage goals. Thank you for taking the time and considering the feature.