Makes the dropdown styles compatible with the pinned state of the
menu so that both states can share the same styles.
- New vector-menu-heading mixin shared between main menu headings
and dropdown headings.
- New @font-size-dropdown variable so font-size is consistent in
dropdown and pinned state.
- Ensure hidden 'more' menu works in both pinned and dropdown state.
- Removes extra <ul><li> from PinnableDropdownContents.mustache, as the
child Menu template already contain <ul><li> elements.
Bug: T318434
Change-Id: Ie1a56d8584c1dc3f20c6643ff4cfc740be4007fa
