Use baked CSS colors instead of opacity for flakiness interpolated results

Chrome does not handle large numbers of layers well and the flakiness
dashboard was creating thousands of them (one for each interpolated
result.) This patch switches to manually calculating the opacity value
and baking it into the css. This is less maintainable but avoids
expensive non-zero opacity layers.

http://hex2rgba.devoth.com is a simple tool I used to calculate the
20% opacity version of the test result colors.

NOTRY=true
TEST=manual. With this patch applied, the flakiness dashboard scrolls smoothly.
BUG=397886

Review URL: https://codereview.chromium.org/421853002

git-svn-id: svn://svn.chromium.org/blink/trunk@178994 bbb929c8-8fbe-4397-9dbb-9b2b20218538