Add normative algorithm for when to call "fetch a manifest image resource" #1216

Problem

PR #1171 introduces a "fetch a manifest image resource" algorithm but leaves it with no normative call site. Issue #910 is about clarifying how manifest image resources are fetched — the algorithm alone doesn't fully address this without specifying when it should be called.

Browser behaviour (verified in source)

A browser impl check against Chrome and Firefox shows:

• Chrome (ManifestToWebAppInstallInfoJob): icons are fetched after manifest parsing/validation, via WebContents::DownloadImageInFrame() with initiator_frame_routing_id, tied to the originating renderer frame. Called during the install flow before showing the install dialog.
• Firefox (Manifest.sys.mjs:prefetch()): icons are fetched just before the install UI is presented, then cached. aWindow.fetch() with mode: "cors" goes through the content window.

Neither browser fetches icons during regular page loads. Both require a live document/window context.

Proposed fix

Add a normative "obtaining manifest icons" step (or similar) within the manifest application/installation flow that calls "fetch a manifest image resource" for each icon. This likely requires first adding a normative installation algorithm to the spec, which is a larger undertaking.

As an interim, PR #1171 adds an informative note on the algorithm explaining when it is intended to be called.

Related

• PR Clarify how manifest image resources are fetched #1171
• Closes Clarify how images are loaded, and that they don't depend on CSP #910 (partially — PR Clarify how manifest image resources are fetched #1171 handles the "how", this handles the "when")
• Set fetch mode to cors for image resource fetching image-resource#50 (fetch mode should be cors)