fix: properly determine if WebContents is offscreen in WebContentsDelegate

[BinaryMuse]

Description of Change

WebContentsDelegate::InitWithWebContents previously used the following code to set offscreen_:

  // Determien whether the WebContents is offscreen.
  auto* web_preferences = WebContentsPreferences::From(web_contents);
  offscreen_ =
      !web_preferences || web_preferences->IsEnabled(options::kOffscreen);

However, options::kOffscreen defaults to false when its not set, so when web_preferences was null, offscreen_ would be set to true. The most immediate result of this is that RunModalDialog would have settings.force_detached set in the following code, causing the dialog to be shown as a regular modal instead of a sheet:

// Run modal dialog with parent window and return user's choice.
int RunModalDialog(NSSavePanel* dialog, const DialogSettings& settings) {
  __block int chosen = NSFileHandlingPanelCancelButton;
  if (!settings.parent_window || !settings.parent_window->GetNativeWindow() ||
      settings.force_detached) {
    chosen = [dialog runModal];
  } else {
    NSWindow* window = settings.parent_window->GetNativeWindow();

    [dialog beginSheetModalForWindow:window
                   completionHandler:^(NSInteger c) {
                     chosen = c;
                     [NSApp stopModal];
                   }];
    [NSApp runModalForWindow:window];
  }

  return chosen;
}

This change defaults offscreen_ to false when web_preferences is null.

/cc @VishwasShashidhar

Fixes #16300

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: Changed the file dialog presented by inputs with the type file to use sheets on macOS instead of detached modals.

[BinaryMuse]

Interestingly, I'm seeing web_preferences being null even though I set webPreferences when I created the BrowserWindow. This seems unexpected to me, but I may be missing something.

[codebytere]

unsure if there's a way to test for this, but it'd be awesome if there way 🤔

[release-clerk]

Release Notes Persisted

Changed the file dialog presented by inputs with the type file to use sheets on macOS instead of detached modals.

[trop]

I have automatically backported this PR to "3-1-x", please check out #16341

[trop]

I have automatically backported this PR to "4-0-x", please check out #16340

[VishwasShashidhar]

Confirmed that it is fixed on master and we see a sheet. Thanks @BinaryMuse @codebytere