Make CssResourceGenerator produce stable and correct output.

In incremental mode CssResourceGenerator produces collisions of CSS class
names, since its obfuscation depends in an internal counter.
We need the generator to produce stable and unique names,
so we are forcing this in incremental compiles.

Change-Id: I636f8d8254961e2b724a9442cd8b5df83067051b
1 file changed