Handle more scale factors for extension Browser Action icons
Also use 16x16 if provided in Material Design mode.
Also, fix size of default icon (generated from extension's first letter) for MD.
BUG=
564926
Committed:
https://crrev.com/39ea51bd9379fa9aaa974f8b91a559879175e681
Cr-Commit-Position: refs/heads/master@{#363278}