webview/plugins: Allow manifest-listed resources to be loaded in plugins.
If a plugin attempts to load a resource that belongs to an extension/app in
a <webview>, then it is normally blocked because of content-settings rules.
However, if the app lists the resource(s) in |accessible_resources| in its
manifest, then allow the plugin to load them. In this case, a
'permissionrequest' event triggers in the embedder, and the app can decide
whether to allow the guest to load the plugin or not.
BUG=422372
R=bauerb@chromium.org, fsamuel@chromium.org
Review URL: https://codereview.chromium.org/
724053006
Cr-Commit-Position: refs/heads/master@{#306817}