Legend/popover colour chip update for bar chart

Problem

From discussion here: gitlab-org/gitlab-services/design.gitlab.com#614 (comment 364661865)

• The color chip in the legend/popover doesn't really match the columns in charts
• It's hard to recognise that you can actually click chips in the legend to show/hide values in chart
• It's current hard to differentiate between lines and columns on the same chart: gitlab-org/gitlab-services/design.gitlab.com#615

Solution

Legend/popover colour chip update for bar chart

Style as described here: gitlab-org/gitlab-services/design.gitlab.com#617 (comment 370810027)