DevTools: NetworkPanel: reduce use of "opacity".

We're going to redesign color scheme to show more information.
This is a preparation step.

This patch makes bars opaque and tunes colors, to preserve old UI.

Screenshots:

Before:
http://picpaste.com/before_2.png

After:
http://picpaste.com/after_3.png


BUG=
NOTRY=true

Review URL: https://codereview.chromium.org/494353002

git-svn-id: svn://svn.chromium.org/blink/trunk@180844 bbb929c8-8fbe-4397-9dbb-9b2b20218538