Rewrite CSSMin::remap to support multiple url() values in one rule

Each can be selectively embedded by placing the /* @embed */ comment
just before the url() value. /* @embed */ at the beginning of the rule
affects all url() values appearing in it.

Three changes in existing behavior for previously supported syntax:
* /* @embed */ comments are no longer preserved in output
* rules not terminated by semicolons are correctly supported
* spaces within url() values are correctly supported

Bug: 46757
Bug: 56514
Change-Id: If9082f553fa920c606f12093f39f4a163ebacc32