Fix CSSMin url() remapping when comments in CSS contain curly braces

Remapping of url()s was thrown off if the CSS contained comments
with curly braces. To fix, replace all block comments (except @embed
directives) with placeholders before url() processing and swap them
back in afterwards. Patch also includes tests.

Bug: 60077
Change-Id: If18a93c17ea9bcd529f6a664aa2dcc51d4a37f38
Merge "Fix CSSMin url() remapping when comments in CSS contain curly braces"