[Extensions UI] Add a placeholder icon for extensions in the toolbar

[Extensions UI] Add a placeholder icon for extensions in the toolbar

Create a placeholder icon source for extensions that don't have an icon, so that
they don't just look like puzzle pieces in the toolbar.
To start, only use this in the toolbar, but the class is flexible enough that
it could also be used, e.g., in the chrome://extensions page.

BUG=486206

Review URL: https://codereview.chromium.org/1131443006

Cr-Commit-Position: refs/heads/master@{#329932}