Fix a bug in spa/chart-legend.
Fix a bug in spa/chart-legend.

chart-section will listen for clicks on both legend leaves and the legend itself
(outside of leaves).
This CL stops leaf clicks from propagating up to the legend itself, so that
chart-section can distinguish leaf clicks from legend clicks.

Bug: chromium:918193
Change-Id: Ia8853fa71d2d0a368517413defbe71bdbcd5515e
Reviewed-on: https://chromium-review.googlesource.com/c/1480855
Reviewed-by: Simon Hatch <490a224d89645d3fea983d7189ce51bde0a00c1d@chromium.org>
Commit-Queue: Ben Hayden <d3ffe0e065c2328e497bb32a07f4dcb29138ef5b@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult