DevTools: Simplify the CSS concatenation approach using an intermediate stylesheet

We used to load all core CSS from inspector.html using the <link> tag.
This required HTML parsing during the CSS concatenation step. This patch
makes inspector.css @import all required stylesheets, and it is parsed
by the concatenation script during the build step.

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/397953003

git-svn-id: svn://svn.chromium.org/blink/trunk@178286 bbb929c8-8fbe-4397-9dbb-9b2b20218538